gnt-cluster modify: check vg name only on vm-capable nodes

This fixes issue 432: when setting the volume group name, the presence
of the volume group was checked on all nodes. This patch now restricts
the check only to the vm-capable ones.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>