monitor: add query-vnc-servers command
Add new query vnc qmp command, for the lack of better ideas just name it
"query-vnc-servers". Changes over query-vnc:
* It returns a list of vnc servers, so multiple vnc server instances
are covered.
* Each vnc server returns a list of server sockets. Followup patch
will use that to also report websockets. In case we add support for
multiple server sockets server sockets (to better support ipv4+ipv6
dualstack) we can add them to the list too.
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>