pvscan: avoid full scan for activation

pvscan: avoid full scan for activation

When an online PV completed a VG, the standard
activation functions were used to activate the VG.
These functions use a full scan of all devs.
When many pvscans are run during startup and need
to activate many VGs, scanning all devs from all
the pvscans can take a long time.

Optimize VG activation in pvscan to scan only the
devs in the VG being activated.  This makes use of
the online file info that was used to determine
the VG was complete.

The downside of this approach is that pvscan activation
will not detect duplicate PVs and block activation,
where a normal activation command (which scans all
devices) would.