Additional fixes for lv_mirror_count.

Additional fixes for lv_mirror_count.

Changing lv_mirror_count to only count the AREA_LVs made the function
stop working for PVMOVE mirrors.  A conditional has been added to fix
that problem.  Additionally, when counting the images in a mirror stack,
we don't need to subtract 1 from the count we get back from the
lv_mirror_count call on the temporary mirror layer.  (This is because we
are no falsely counting the top layer of the temporary mirror.)