monitor: add query-vnc-servers command

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>