qapi: implement block-dirty-bitmap-remove transaction action

qapi: implement block-dirty-bitmap-remove transaction action

It is used to do transactional movement of the bitmap (which is
possible in conjunction with merge command). Transactional bitmap
movement is needed in scenarios with external snapshot, when we don't
want to leave copy of the bitmap in the base image.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Signed-off-by: John Snow <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-id: 20190708220502[email protected]
[Edited "since" version to 4.2 --js]
Signed-off-by: John Snow <[email protected]>