Rename pvbus component to pv to allow conditional NPV define

pvbus is always defined in files.pv, which prevents the use of
#if NPVBUS > 0. Renaming it to pv aligns with naming conventions used
for other bus components and allows NPV to be conditionally defined.

Add support for command line MMIO devices, at least qemu and
Firecracker pass MMIO virtual devices mapping through the kernel
command line.
This driver is based on Colin Percival's FreeBSD virtio_mmio_cmdline.c
https://github.com/freebsd/freebsd-src/blob/main/sys/dev/virtio/mmio/virtio_mmio_cmdline.c

The following kernel options are needed

options MPBIOS
options MPTABLE_LINUX_BUG_COMPAT

As are these drivers

pv* at pvbus?
virtio* at pv?

Exemple qemu usage on a Linux host to boot a NetBSD guest:

qemu-system-x86_64 \
-M microvm,x-option-roms=off,rtc=on,acpi=off,pic=off,accel=kvm \
-m 256 -cpu host -kernel ${KERNEL} \
-append "root=ld0a console=com rw -v" \
-device virtio-blk-device,drive=hd0 \
-drive file=${IMG},format=raw,id=hd0 \
-device virtio-net-device,netdev=net0 \
-netdev user,id=net0,ipv6=off,hostfwd=::2200-:22 \
-global virtio-mmio.force-legacy=false -display none -serial stdio

A lightweight kernel configuration named MICROVM if available for this
use case.

Trivial bus implementation inspired by OpenBSD's pv(4) to attach devices
that don't need nor rely on a PCI or ISA bus.

x86/pvbus(4): KNF. No functional change intended.

Trivial bus implementation inspired by OpenBSD's pv(4) to attach devices
that don't need nor rely on a PCI or ISA bus.

Trivial bus implementation inspired by OpenBSD's pv(4) to attach devices
that don't need nor rely on a PCI or ISA bus.