Migrate call from backend._GetVGInfo to bdev.LogicalVolume.GetVGInfo

This patch removes duplicate code found in backend which also needs to
get VG infos. To make it simpler we moved to bdev.LogicalVolume.GetVGInfo.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>