Change bdev.LogicalVolume.GetPVInfo usage

We will need to enumerate selectively the PVs of (possible) many VGs and
not only the allocatable ones. For this we make the VG selection and the
allocatable filtering optional. The two callers are modified for this
new calling syntax.

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