[PowerPC] Add virtio to GENERIC

Due to kldxref not being able to generate hints for nonnative platforms,
any cross built VM images do not have /boot/kernel/linker.hints.

This prevents the virtio modules from being loaded, as the fallback code
will always fail the version check when the hints are missing.

Since we want to be able to generate VM images for 32 bit powerpc, add the
virtio modules to GENERIC like we do on powerpc64.

Reviewed by: jhibbits
Sponsored by: Tag1 Consulting, Inc.
Differential Revision: https://reviews.freebsd.org/D25271