vnc: auto assian an id when calling change vnc qmp interface

Only in this way, change vnc qmp interface can take effect,
because qemu_opts_find(&qemu_vnc_opts, id) will return NULL
in vnc_display_open(), It can't connect successfully vnc
server forever.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>