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 <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

jfehlig/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,nertpinx/libvirt,jardasgit/libvirt,jardasgit/libvirt,olafhering/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,crobinso/libvirt,olafhering/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,zippy2/libvirt,zippy2/libvirt,zippy2/libvirt,libvirt/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,jardasgit/libvirt,crobinso/libvirt,olafhering/libvirt,zippy2/libvirt,jardasgit/libvirt,libvirt/libvirt,crobinso/libvirt,nertpinx/libvirt,nertpinx/libvirt,jfehlig/libvirt,nertpinx/libvirt,libvirt/libvirt,libvirt/libvirt,fabianfreyer/libvirt,crobinso/libvirt,olafhering/libvirt