block/nbd: fix segmentation fault when .desc is not null-terminated

The find_desc_by_name() from util/qemu-option.c relies on the .name not being
NULL to call strcmp(). This check becomes unsafe when the list is not
NULL-terminated, which is the case of nbd_runtime_opts in block/nbd.c, and can
result in segmentation fault when strcmp() tries to access an invalid memory:

    #0 0x00007fff8c75f7d4 in __strcmp_power9 () from /lib64/libc.so.6
    #1 0x00000000102d3ec8 in find_desc_by_name (desc=0x1036d6f0, name=0x28e46670 "server.path") at util/qemu-option.c:166
    #2 0x00000000102d93e0 in qemu_opts_absorb_qdict (opts=0x28e47a80, qdict=0x28e469a0, errp=0x7fffec247c98) at util/qemu-option.c:1026
    #3 0x000000001012a2e4 in nbd_open (bs=0x28e42290, options=0x28e469a0, flags=24578, errp=0x7fffec247d80) at block/nbd.c:406
    #4 0x00000000100144e8 in bdrv_open_driver (bs=0x28e42290, drv=0x1036e070 <bdrv_nbd_unix>, node_name=0x0, options=0x28e469a0, open_flags=24578, errp=0x7fffec247f50) at block.c:1135
    #5 0x0000000010015b04 in bdrv_open_common (bs=0x28e42290, file=0x0, options=0x28e469a0, errp=0x7fffec247f50) at block.c:1395

>From gdb, the desc[i].name was not NULL and resulted in strcmp() accessing an
invalid memory:

    >>> p desc[5]
    $8 = {
      name = 0x1037f098 "R27A",
      type = 1561964883,
      help = 0xc0bbb23e <error: Cannot access memory at address 0xc0bbb23e>,
      def_value_str = 0x2 <error: Cannot access memory at address 0x2>
    }
    >>> p desc[6]
    $9 = {
      name = 0x103dac78 <__gcov0.do_qemu_init_bdrv_nbd_init> "\001",
      type = 272101528,
      help = 0x29ec0b754403e31f <error: Cannot access memory at address 0x29ec0b754403e31f>,
      def_value_str = 0x81f343b9 <error: Cannot access memory at address 0x81f343b9>
    }

This patch fixes the segmentation fault in strcmp() by adding a NULL element at
the end of nbd_runtime_opts.desc list, which is the common practice to most of
other structs like runtime_opts in block/null.c. Thus, the desc[i].name != NULL
check becomes safe because it will not evaluate to true when .desc list reached
its end.

Reported-by: R. Nageswara Sastry <nasastry@in.ibm.com>
Buglink: https://bugs.launchpad.net/qemu/+bug/1727259
Signed-off-by: Murilo Opsfelder Araujo <muriloo@linux.vnet.ibm.com>
Message-Id: <20180105133241.14141-2-muriloo@linux.vnet.ibm.com>
CC: qemu-stable@nongnu.org
Fixes: 7ccc44fd7d1dfa62c4d6f3a680df809d6e7068ce
Signed-off-by: Eric Blake <eblake@redhat.com>
