[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

share/man/man4/Makefile

@@ -868,8 +868,16 @@ _nvram2env.4=	nvram2env.4
 .endif
 
 .if ${MACHINE_CPUARCH} == "powerpc"
+_if_vtnet.4=    if_vtnet.4
 _nvd.4= 	nvd.4
 _nvme.4=	nvme.4
+_virtio.4=	virtio.4
+_virtio_balloon.4=virtio_balloon.4
+_virtio_blk.4=	virtio_blk.4
+_virtio_console.4=virtio_console.4
+_virtio_random.4= virtio_random.4
+_virtio_scsi.4=	virtio_scsi.4
+_vtnet.4=	vtnet.4
 .endif
 
 .if empty(MAN_ARCH)

sys/modules/Makefile

@@ -761,6 +761,7 @@ _nvd=		nvd
 _nvme=		nvme
 _pccard=	pccard
 _wi=		wi
+_virtio=	virtio
 .endif
 
 .if ${MACHINE_ARCH} == "powerpc64"

sys/powerpc/conf/GENERIC64

@@ -256,3 +256,12 @@ device		netmap		# netmap(4) support
 options 	EVDEV_SUPPORT		# evdev support in legacy drivers
 device		evdev			# input event device support
 device		uinput			# install /dev/uinput cdev
+
+# VirtIO support
+device          virtio                  # Generic VirtIO bus (required)
+device          virtio_pci              # VirtIO PCI device
+device          vtnet                   # VirtIO Ethernet device
+device          virtio_blk              # VirtIO Block device
+device          virtio_scsi             # VirtIO SCSI device
+device          virtio_balloon          # VirtIO Memory Balloon device
+