qemu: Revoke access to mirror on failed blockcopy

qemu: Revoke access to mirror on failed blockcopy

When preparing to do a blockcopy, the mirror image is modified so
that QEMU can access it. For instance, the mirror has seclabels
set, if it is a NVMe disk it is detached from the host and so on.
And usually, the restore is done upon successful finish of the
blockcopy operation. But, if something fails then we need to
explicitly revoke the access to the mirror image (and thus
reattach NVMe disk back to the host).

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1822538
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Pavel Mores <pmores@redhat.com>