[PowerPC64] Enable virtio drivers

This enables virtio modules on PowerPC* target.
On PowerPC64, drivers are also kernel builtin.

QEMU currently needs to be patched to in order to work on LE hosts due to known
issue affecting pre-1.0 (legacy) virtio drivers.

The patch was submitted to QEMU mail list by @afscoelho_gmail.com, available at
https://lists.nongnu.org/archive/html/qemu-devel/2020-01/msg01496.html

Submitted by: Alfredo Dal'Ava Junior <alfredo.junior@eldorado.org.br>
Reviewed by: luporl
Differential Revision: https://reviews.freebsd.org/D22833