Introduce a 'client_add' monitor command accepting an open FD

Allow client connections for VNC and socket based character
devices to be passed in over the monitor using SCM_RIGHTS.

One intended usage scenario is to start QEMU with VNC on a
UNIX domain socket. An unprivileged user which cannot access
the UNIX domain socket, can then connect to QEMU's VNC server
by passing an open FD to libvirt, which passes it onto QEMU.

 { "execute": "get_fd", "arguments": { "fdname": "myclient" } }
 { "return": {} }
 { "execute": "add_client", "arguments": { "protocol": "vnc",
                                           "fdname": "myclient",
                                           "skipauth": true } }
 { "return": {} }

In this case 'protocol' can be 'vnc' or 'spice', or the name
of a character device (eg from -chardev id=XXXX)

The 'skipauth' parameter can be used to skip any configured
VNC authentication scheme, which is useful if the mgmt layer
talking to the monitor has already authenticated the client
in another way.

* console.h: Define 'vnc_display_add_client' method
* monitor.c: Implement 'client_add' command
* qemu-char.c, qemu-char.h: Add 'qemu_char_add_client' method
* qerror.c, qerror.h: Add QERR_ADD_CLIENT_FAILED
* qmp-commands.hx: Declare 'client_add' command
* ui/vnc.c: Implement 'vnc_display_add_client' method

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
