use vdsm to get storage path only for nfs

Change-Id: I3e9909ae70a1f3d5545c8c05ebc8b210a68ee642
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1150906
Signed-off-by: Jiri Moskovcak <jmoskovc@redhat.com>