virtio: Reduce boilerplate for device driver module definitions

Rather than have every device register itself for both virtio_pci and
virtio_mmio, provide a VIRTIO_DRIVER_MODULE wrapper to declare both,
merge VIRTIO_SIMPLE_PNPTABLE with VIRTIO_SIMPLE_PNPINFO and make the
latter register for both buses. This also has the benefit of abstracting
away the available transports and their names.

Reviewed by: bryanv
Differential Revision: https://reviews.freebsd.org/D28073