gnt-cluster verify-disks: fix VG name

Recent multi-VG work already exports the missing LV names as vg/lv,
not simply lv. So the query and addition of the VG name in gnt-cluster
verify-disks is redundant, and even wrong for non-default-VG
instances.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>