qemu: Rewrite bitmap handling for block commit

qemu: Rewrite bitmap handling for block commit

Reuse qemuBlockGetBitmapMergeActions which allows removing the ad-hoc
implementation of bitmap merging for block commit. The new approach is
way simpler and more robust and also allows us to get rid of the
disabling of bitmaps done prior to the start as we actually do want to
update the bitmaps in the base.

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