[PPC64] Support QEMU/KVM pseries without hugepages

This set of changes make it possible to run FreeBSD for PowerPC64/pseries,
under QEMU/KVM, without requiring the host to make hugepages available to the
guest.

While there was already this possibility, by means of setting hw_direct_map to
0, on PowerPC64 there were a couple of issues/wrong assumptions that prevented
this from working, before this changelist.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D20522