lvm: Modify lv selinux label only if not labablled as libvirt image

When using the faulty version of systemd that removes libvirt image
labels from block devices, this patch has no effect. However when a fix
is available and libvirt image label exists, vdsm will not change the
original libvirt label. This allows the increase protection for virtual
machines.

Change-Id: Ide7560564e4c83c84dd288b5a8305ad1ddb4cfcb
Bug-Url: https://bugzilla.redhat.com/1127460
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33620
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>