Fix handling of partial VG for lvm1 format metadata

Fix handling of partial VG for lvm1 format metadata

If some lvm1 device is missing, lvm fails on all operations
# vgcfgbackup -f bck -P vg_test
  Partial mode. Incomplete volume groups will be activated read-only.
  3 PV(s) found for VG vg_test: expected 4
  PV segment VG free_count mismatch: 152599 != 228909
  PV segment VG extent_count mismatch: 152600 != 228910
  Internal error: PV segments corrupted in vg_test.
  Volume group "vg_test" not found

Allow loading of lvm1 partial VG by allocating "new" missing PV,
which covers lost space. Also this fake mising PV inform code
that it is partial VG.

https://bugzilla.redhat.com/show_bug.cgi?id=501390