storage: Fix VMPathNotExists text

Change-Id: I6b374a818be18340e2b9fc5d91ba8e0d4f694f6f
Bug-Url: https://bugzilla.redhat.com/1169512
Signed-off-by: Allon Mureinik <amureini@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35880
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>