Add migrate_incoming/migrate-incoming to start an incoming
migration.

Once a qemu has been started with
    -incoming defer

the migration can be started by issuing:
    migrate_incoming uri

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>