Update check in vg_split_mdas to account for ignored mdas list.

Update check in vg_split_mdas to account for ignored mdas list.

The check in vg_split_mdas will trigger an error if the 'from' vg
list is empty.  However, this might be ok in some instances now
that we have ignored mdas.  Relax this check so an error is triggered
only in the case where there's truly no more mdas in the 'from'
vg.

One example of where this makes a difference is with vgreduce.
If we try to vgreduce a PV with un-ignored mdas, this should trigger
the balancing function to un-ignore mdas on another PV in the VG.
However, we don't get to vg_write() before we fail because this
list size check fails, and we see an error message indicating:
"Cannot remove final metadata area ..."

Another example is with vgsplit into a new VG, where the PVs
being moved contain all ignored mdas.  We must move the mdas on
fid->metadata_areas_ignored from 'vg_from' to 'vg_to'.

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