[rhel6-branch] two more fixes for /boot on LVM on s390x

[David Cantrell]

1) Prevent /boot lv requests on s390 from being larger than any available
   pv in the vg.

2) Prevents users from installing on a system with only / as an lv.  We
   need /boot as an lv or regular partition -or- / as a regular partition
   on s390x.

For master as well.