virtio_mmio: Improve V1 spec conformance

Implement the virtio_bus_finalize_features method so the FEATURES_OK
status bit is set. Implement the virtio_bus_config_generation method
to ensure larger than 4-byte reads are consistent.

Reviewed by: cperciva
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36150