pvchange, pvresize: move exported VG check

pvchange, pvresize: move exported VG check

Allow pvchange and pvresize to process exported VGs,
and have them check for the exported state in their
single function.

Previously, the exported VG state would trigger a
failure in vg_read()/ignore_vg() because the VGs are
being read with READ_FOR_UPDATE.  Because these commands
read all VGs to search for the intended PVs, any
exported VG would trigger a failure, even if it was
not related to the intended PV.