virtio(4): Expose PNP metadata through newbus

Expose the same fields and widths from both vtio buses, even though they
don't quite line up; several virtio drivers can attach to both buses,
and sharing a PNP info table for both seems more convenient.

In practice, I doubt any virtio driver really needs to match on anything
other than bus and device_type (eliminating the unused entries for
vtmmio), and also in practice device_type is << 2^16 (so far, values
range from 1 to 20). So it might be fine to only expose a 16-bit
device_type for PNP purposes. On the other hand, I don't see much harm
in overkill here.

Reviewed by: bryanv, markj (earlier version)
Differential Revision: https://reviews.freebsd.org/D20406