qemuProcessStop: Remove image metadata for running mirror jobs

If user starts a blockcommit or a blockcopy then we modify access
for qemu on both images and leave it like that until the job
terminates. So far so good. Problem is, if user instead of
terminating the job (where we would modify the access again so
that the state before the job is restored) calls destroy on the
domain or if qemu dies whilst executing the block job. In this
case we don't ever clear the access we granted at the beginning.
To fix this, maybe a bit harsh approach is used, but it works:
after all labels were restored (that is after
qemuSecurityRestoreAllLabel() was called), we iterate over each
disk in the domain and remove XATTRs from the whole backing chain
and also from any file the disk is being mirrored to.

This would have been done at the time of pivot, but it isn't
because user decided to kill the domain instead. If we don't do
this and leave some XATTRs behind the domain might be unable to
start.

Also, secdriver can't do this because it doesn't know if there is
any job running. It's outside of its scope - the hypervisor
driver is responsible for calling secdriver's APIs.

Moreover, this is safe to call because we don't remember labels
for any member of a backing chain except of the top layer. But
that one was restored in qemuSecurityRestoreAllLabel() call done
earlier. Therefore, not only we don't remember labels (and thus
this is basically a NOP for other images in the backing chain) it
is also safe to call this when no blockjob was started in the
first place, or if some parts of the backing chain are shared
with some other domains - this is NOP, unless a block job is
active at the time of domain destroy.

https://bugzilla.redhat.com/show_bug.cgi?id=1741456#c19

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>