toollib: skip PV if system ID is used and PV marked as used but metadata missing

toollib: skip PV if system ID is used and PV marked as used but metadata missing

If we know that a PV belongs to some VG and we're missing metadata
(because we have only those PV(s) from VG present in the system that
don't have metadata areas), we should skip such PV when processing
under system ID.

This is because we know that the PV belongs to some VG, but we
really can't decide whether it matches system ID unless the VG
metadata is present again.