Make snapshots multi-vg aware

Currently, the Snapshot() function of LogicalVolume returns only the
logical volume path, with the assumption that we only have one VG. But
with the recent changes, it makes more sense to return the full data (vg
and lv) from it, so as to not require computing it in the master.

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