sos: plugin should ignore /var/run/vdsm/storage

In case sos plugin includes /var/run/vdsm/storage and it contains
symlinks to block devices it can make the host fill the disk
and eventually create a 0 sized sosreport.

Change-Id: Ie60d681cc744f2407a0ddf9406225e73497ac648
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23925
Reviewed-by: Dan Kenigsberg <danken@redhat.com>