patch review: pvbus

Hi,

as part of the performance patch series I've been working on, I'd
like some more review on this new bus before importing it into
current: https://imil.net/NetBSD/pvbus.patch

The purpose of this new bus is to attach devices that don't need nor
rely on PCI bus, like MMIO https://github.com/NetBSD/src/commit/144b37e53da256029377956ecd6f2cb793af3dc4
or pvclock https://github.com/NetBSD/src/commit/6b663f77734395b837d23c1876578939e96e2bab
Both are upcoming patches from the same series.

This is at least needed for VMMs like qemu microvm machine type and
AWS's Firecracker, which don't implement PCI and only rely on MMIO
devices.

Thanks,

------------------------------------------------------------------------
Emile `iMil' Heitor <imil@{home.imil.net,NetBSD.org}> | https://imil.net