Reuse qemuBlockGetBitmapMergeActions which allows removal of the ad-hoc
implementation of bitmap merging for backup. The new approach is simpler
and also more robust in case some of the bitmaps break as they remove
the dependency on the whole chain of bitmaps working.

The new approach also allows backups if a snapshot is created outside of
libvirt.

Additionally the code is greatly simplified.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>