qmp: transaction support for x-block-dirty-bitmap-enable/disable

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 20180606182449.1607-4-jsnow@redhat.com
[Added x- prefix. --js]
Signed-off-by: John Snow <jsnow@redhat.com>