net: don't poke at chardev internal QemuOpts

The vhost-user & colo code is poking at the QemuOpts instance
in the CharDriverState struct, not realizing that it is valid
for this to be NULL. e.g. the following crash shows a codepath
where it will be NULL:

 Program terminated with signal SIGSEGV, Segmentation fault.
 #0  0x000055baf6ab4adc in qemu_opt_foreach (opts=0x0, func=0x55baf696b650 <net_vhost_chardev_opts>, opaque=0x7ffc51368c00, errp=0x7ffc51368e48) at util/qemu-option.c:617
 617         QTAILQ_FOREACH(opt, &opts->head, next) {
 [Current thread is 1 (Thread 0x7f1d4970bb40 (LWP 6603))]
 (gdb) bt
 #0  0x000055baf6ab4adc in qemu_opt_foreach (opts=0x0, func=0x55baf696b650 <net_vhost_chardev_opts>, opaque=0x7ffc51368c00, errp=0x7ffc51368e48) at util/qemu-option.c:617
 #1  0x000055baf696b7da in net_vhost_parse_chardev (opts=0x55baf8ff9260, errp=0x7ffc51368e48) at net/vhost-user.c:314
 #2  0x000055baf696b985 in net_init_vhost_user (netdev=0x55baf8ff9250, name=0x55baf879d270 "hostnet2", peer=0x0, errp=0x7ffc51368e48) at net/vhost-user.c:360
 #3  0x000055baf6960216 in net_client_init1 (object=0x55baf8ff9250, is_netdev=true, errp=0x7ffc51368e48) at net/net.c:1051
 #4  0x000055baf6960518 in net_client_init (opts=0x55baf776e7e0, is_netdev=true, errp=0x7ffc51368f00) at net/net.c:1108
 #5  0x000055baf696083f in netdev_add (opts=0x55baf776e7e0, errp=0x7ffc51368f00) at net/net.c:1186
 #6  0x000055baf69608c7 in qmp_netdev_add (qdict=0x55baf7afaf60, ret=0x7ffc51368f50, errp=0x7ffc51368f48) at net/net.c:1205
 #7  0x000055baf6622135 in handle_qmp_command (parser=0x55baf77fb590, tokens=0x7f1d24011960) at /path/to/qemu.git/monitor.c:3978
 #8  0x000055baf6a9d099 in json_message_process_token (lexer=0x55baf77fb598, input=0x55baf75acd20, type=JSON_RCURLY, x=113, y=19) at qobject/json-streamer.c:105
 #9  0x000055baf6abf7aa in json_lexer_feed_char (lexer=0x55baf77fb598, ch=125 '}', flush=false) at qobject/json-lexer.c:319
 #10 0x000055baf6abf8f2 in json_lexer_feed (lexer=0x55baf77fb598, buffer=0x7ffc51369170 "}R\204\367\272U", size=1) at qobject/json-lexer.c:369
 #11 0x000055baf6a9d13c in json_message_parser_feed (parser=0x55baf77fb590, buffer=0x7ffc51369170 "}R\204\367\272U", size=1) at qobject/json-streamer.c:124
 #12 0x000055baf66221f7 in monitor_qmp_read (opaque=0x55baf77fb530, buf=0x7ffc51369170 "}R\204\367\272U", size=1) at /path/to/qemu.git/monitor.c:3994
 #13 0x000055baf6757014 in qemu_chr_be_write_impl (s=0x55baf7610a40, buf=0x7ffc51369170 "}R\204\367\272U", len=1) at qemu-char.c:387
 #14 0x000055baf6757076 in qemu_chr_be_write (s=0x55baf7610a40, buf=0x7ffc51369170 "}R\204\367\272U", len=1) at qemu-char.c:399
 #15 0x000055baf675b3b0 in tcp_chr_read (chan=0x55baf90244b0, cond=G_IO_IN, opaque=0x55baf7610a40) at qemu-char.c:2927
 #16 0x000055baf6a5d655 in qio_channel_fd_source_dispatch (source=0x55baf7610df0, callback=0x55baf675b25a <tcp_chr_read>, user_data=0x55baf7610a40) at io/channel-watch.c:84
 #17 0x00007f1d3e80cbbd in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
 #18 0x000055baf69d3720 in glib_pollfds_poll () at main-loop.c:213
 #19 0x000055baf69d37fd in os_host_main_loop_wait (timeout=126000000) at main-loop.c:258
 #20 0x000055baf69d38ad in main_loop_wait (nonblocking=0) at main-loop.c:506
 #21 0x000055baf676587b in main_loop () at vl.c:1908
 #22 0x000055baf676d3bf in main (argc=101, argv=0x7ffc5136a6c8, envp=0x7ffc5136a9f8) at vl.c:4604
 (gdb) p opts
 $1 = (QemuOpts *) 0x0

The crash occurred when attaching vhost-user net via QMP:

{
    "execute": "chardev-add",
    "arguments": {
        "id": "charnet2",
        "backend": {
            "type": "socket",
            "data": {
                "addr": {
                    "type": "unix",
                    "data": {
                        "path": "/var/run/openvswitch/vhost-user1"
                    }
                },
                "wait": false,
                "server": false
            }
        }
    },
    "id": "libvirt-19"
}
{
    "return": {

    },
    "id": "libvirt-19"
}
{
    "execute": "netdev_add",
    "arguments": {
        "type": "vhost-user",
        "chardev": "charnet2",
        "id": "hostnet2"
    },
    "id": "libvirt-20"
}

Code using chardevs should not be poking at the internals of the
CharDriverState struct. What vhost-user wants is a chardev that is
operating as reconnectable network service, along with the ability
to do FD passing over the connection. The colo code simply wants
a network service. Add a feature concept to the char drivers so
that chardev users can query the actual features they wish to have
supported. The QemuOpts member is removed to prevent future mistakes
in this area.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
