Fix pvcreate device check.

Fix pvcreate device check.

If user try to vgcreate or vgextend non-existent VG,
these messages appears:

# vgcreate xxx /dev/xxx
  Internal error: Volume Group xxx was not unlocked
  Device /dev/xxx not found (or ignored by filtering).
  Unable to add physical volume '/dev/xxx' to volume group 'xxx'.
  Internal error: Attempt to unlock unlocked VG xxx.

(the same with existing VG and non-existing PV & vgextend)
# vgextend vg_test /dev/xxx
...

It is caused because code tries to "refresh" cache if
md filter is switched on using cache destroy.

But we can change filters and rescan even without this
machinery now, just use refresh_filters
(and reset md filter afterwards).

(Patch also  discovers cache alias bug in vgsplit test,
fix it by using better filter line.)