Only use byte register access in legacy virtio pci

Some simulators don't implement arbitrary sized memory access to the
virtio PCI registers. Follow Linux and use single byte accesses to read
and write to these registers.

Reviewed by: bryanv, emaste (previous version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31424

(cherry picked from commit 89c085b8993d7d1e7b137f99fa6df94c37c3a68a)