qmp: Add announce-self command

Add a qmp command that can trigger guest announcements.

It uses its own announce-timer instance, and parameters
passed to it explicitly in the command.

Like most qmp commands, it's in the main thread/bql, so
there's no racing with any outstanding timer.

Based on work of Germano Veit Michel <germano@redhat.com> and
                 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>