qemuDomainBlockPivot: Handle merging of bitmaps when pivoting an active block-commit

qemuDomainBlockPivot: Handle merging of bitmaps when pivoting an active block-commit

Active layer block commit makes the 'base' image the new top image of
the disk after it finishes. This means that all bitmap operations need
to be handled prior to this happening as we'd lose writes otherwise.

The ideal place is to handle it when pivoting to the new image as only
guest-writes would be happening after this point.

Use qemuBlockBitmapsHandleCommitFinish to calculate the merging
transaction.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>