_slot_operator_update_probe: fix bug #528610

This fixes a case inside _slot_operator_update_probe where it would
select an inappropriate replacement_parent of a lower version than
desired. The problem is solved by rejecting replacement_parent if its
version is lower than the existing parent, and a downgrade is not
desired.

X-Gentoo-Bug: 528610
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=528610