virtio(4): Add PNP match metadata for virtio devices

Register MODULE_PNP_INFO for virtio devices using the newbus PNP information
provided by the previous commit. Matching can be quite simple; existing
probe routines only matched on bus (implicit) and device_type. The same
matching criteria are retained exactly, but is now also available to
devmatch(8).

Reviewed by: bryanv, markj; imp (earlier version)
Differential Revision: https://reviews.freebsd.org/D20407