qapi: Convert query-vnc

qapi: Convert query-vnc

There are three important remarks in relation to the non-qapi command:

 1. This commit also fixes the behavior of the 'query-vnc' and 'info vnc'
    commands to return an error when qemu is built without VNC support
    (ie. --disable-vnc). The non-qapi command would return the OK
    response in QMP and no response in HMP

 2. The qapi version explicitly marks the fields 'host', 'family',
    'service' and 'auth' as optional. Their are not documented as optional
    in the non-qapi command doc, but they would not be returned if
    vnc support is disabled. The qapi version maintains the same
    semantics, but documents those fields correctly

 3. The 'clients' field, which is a list, is marked as optional but is
    always returned. If there are no clients connected an empty list
    is returned. This is not the Right Way to this in the qapi but it's
    how the non-qapi command used to work

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>