block/snapshot: dirty all dirty bitmaps on snapshot-switch

Snapshot-switch actually changes active state of disk so it should
reflect on dirty bitmaps. Otherwise next incremental backup using
these bitmaps will be invalid.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20171023092945.54532-1-vsementsov@virtuozzo.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>