qemu: snapshot: Propagate active bitmaps through external snapshots

qemu: snapshot: Propagate active bitmaps through external snapshots

Re-create any active persistent bitmap in the snapshot overlay image so
that tracking for a checkpoint is persisted. While this basically
duplicates data in the allocation map it's currently the only possible
way as qemu can't mirror the allocation map into a dirty bitmap if we'd
ever want to do a backup.

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