devices: improve handling of duplicate PVs

devices: improve handling of duplicate PVs

Example:

/dev/loop0 and /dev/loop1 are duplicates,
created by copying one backing file to the
other.

'identity /dev/loopX' creates an identity
mapping for loopX named idmloopX, which
adds a duplicate for the named device.

The duplicate selection code for lvmetad is
incomplete, and lvmetad is disabled for this
example.

[~]# losetup -f loopfile0
[~]# pvs
  PV         VG           Fmt  Attr PSize   PFree
  /dev/loop0 foo          lvm2 a--  308.00m 296.00m

[~]# losetup -f loopfile1
[~]# pvs
  Found duplicate PV LnSOEqzEYED3RvIOa5PZP2s7uyuBLmAV: using /dev/loop1 not /dev/loop0
  Using duplicate PV /dev/loop1 which is more recent, replacing /dev/loop0
  PV         VG           Fmt  Attr PSize   PFree
  /dev/loop1 foo          lvm2 a--  308.00m 308.00m

[~]# ./identity /dev/loop0
[~]# pvs
  Found duplicate PV LnSOEqzEYED3RvIOa5PZP2s7uyuBLmAV: using /dev/loop1 not /dev/loop0
  Using duplicate PV /dev/loop1 without holders, replacing /dev/loop0
  Found duplicate PV LnSOEqzEYED3RvIOa5PZP2s7uyuBLmAV: using /dev/mapper/idmloop0 not /dev/loop1
  Using duplicate PV /dev/mapper/idmloop0 from subsystem DM, replacing /dev/loop1
  PV                   VG           Fmt  Attr PSize   PFree
  /dev/mapper/idmloop0 foo          lvm2 a--  308.00m 296.00m

[~]# ./identity /dev/loop1
[~]# pvs
  WARNING: duplicate PV LnSOEqzEYED3RvIOa5PZP2s7uyuBLmAV is being used from both devices /dev/loop0 and /dev/loop1
  Found duplicate PV LnSOEqzEYED3RvIOa5PZP2s7uyuBLmAV: using /dev/loop1 not /dev/loop0
  Using duplicate PV /dev/loop1 which is more recent, replacing /dev/loop0
  Found duplicate PV LnSOEqzEYED3RvIOa5PZP2s7uyuBLmAV: using /dev/mapper/idmloop0 not /dev/loop1
  Using duplicate PV /dev/mapper/idmloop0 from subsystem DM, replacing /dev/loop1
  Found duplicate PV LnSOEqzEYED3RvIOa5PZP2s7uyuBLmAV: using /dev/mapper/idmloop1 not /dev/mapper/idmloop0
  Using duplicate PV /dev/mapper/idmloop1 which is more recent, replacing /dev/mapper/idmloop0
  PV                   VG           Fmt  Attr PSize   PFree
  /dev/mapper/idmloop1 foo          lvm2 a--  308.00m 308.00m