qapi: add query-display-options command

qapi: add query-display-options command

Add query-display-options command, which allows querying the qemu
display configuration.  This isn't particularly useful, except it
exposes QAPI type DisplayOptions in query-qmp-schema, so that libvirt
can discover recently added -display parameter rendernode (commit
d4dc4ab133b).  Works around lack of sufficiently powerful command line
introspection.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
Tested-by: Erik Skultety <eskultet@redhat.com>
Message-id: 20181122071613.2889-1-kraxel@redhat.com

[ kraxel: reworded commit message as suggested by armbru ]