hmp: Add hmp_announce_self

Add an HMP command to trigger self annocements.
Unlike the QMP command (which takes a set of parameters), the HMP
command reuses the set of parameters used for migration.

Signend-off-by: Vladislav Yasevich <vyasevic@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>