pvs: fix missing PVs when VG is removed

pvs: fix missing PVs when VG is removed

PVs could be missing from the 'pvs' output if
their VG was removed at the same time that the
'pvs' command was run.  To fix this:

1. If a VG is not found when processed, don't
silently skip the PVs in it, as is done when
the "skip" variable is set.

2. Repeat the VG search if some PVs are not
found on the first search through all VGs.
The second search uses a specific list of
PVs that were missed the first time.

testing:
/dev/sdb is a PV
/dev/sdd is a PV
/dev/sdg is not a PV

each test begins with:
vgcreate test /dev/sdb /dev/sdd

variations to test:
vgremove -f test & pvs
vgremove -f test & pvs -a
vgremove -f test & pvs /dev/sdb /dev/sdd
vgremove -f test & pvs /dev/sdg
vgremove -f test & pvs /dev/sdb /dev/sdg

The pvs command should always display /dev/sdb
and /dev/sdd, either as a part of VG test or not.

The pvs command should always print an error
indicating that /dev/sdg could not be found.