Add del_pvl_from_vgs() and move prototypes into metadata-exported.h

Add del_pvl_from_vgs() and move prototypes into metadata-exported.h

Add a delete function to manage the vg->pvs list.

NOTE: It may be possible to do further cleanup to these add/del functions
by passing a 'pv' as input instead of 'pv_list'.  The pv_list is used for
functions which do allocations (lvcreate) while other places in the code
just manage a list of 'pv' (e.g. import functions, vgextend, etc).

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>