Add mda_locns_match() internal library function for mapping pv/device to VG mda.

Add mda_locns_match() internal library function for mapping pv/device to VG mda.

A metadata_area is defined independent of the location.  One downside
is that there is no obvious mapping from a pv to an mda.  For a PV in
a VG, we need a way to start with a PV and end up with an MDA, if we
are to manage mdas starting with a device/pv.  This function provides
us a way to go down the list of PVs on a VG, and identify which ones
match a particular PV.

I'm not entirely happy with this approach, but it does fit into the
existing structures in a reasonable way.

An alternative solution might be to refactor the VG - PV interface such
that mdas are a list tied to a PV.  However, this seemed a bit tricky since
a PV does not come into existence until after the list of mdas is
constructed (see _vg_read() - we create a 'fid' and attach mdas to it,
then we go through them and attach pvs).

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Reviewed-by: Alasdair G Kergon <agk@redhat.com>