When preparing images for block jobs we modify their seclabels so
that QEMU can open them. However, as mentioned in the previous
commit, secdrivers base some it their decisions whether the image
they are working on is top of of the backing chain. Fortunately,
in places where we call secdrivers we know this and the
information can be passed to secdrivers.

The problem is the following: after the first blockcommit from
the base to one of the parents the XATTRs on the base image are
not cleared and therefore the second attempt to do another
blockcommit fails. This is caused by blockcommit code calling
qemuSecuritySetImageLabel() over the base image, possibly
multiple times (to ensure RW/RO access). A naive fix would be to
call the restore function. But this is not possible, because that
would deny QEMU the access to the base image.  Fortunately, we
can use the fact that seclabels are remembered only for the top
of the backing chain and not for the rest of the backing chain.
And thanks to the previous commit we can tell secdrivers which
images are top of the backing chain.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1803551Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NPeter Krempa <pkrempa@redhat.com>