Only use byte register access in legacy virtio pci

[andrew]

Summary

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.

[emaste]

LGTM

[bryanv]

sys/dev/virtio/pci/virtio_pci_legacy.c

514–516

Nit: IMO these for loops end up being a little more complicated to read now that the body is simpler. For this one, perhaps something like:

for (d = dst, i = 0; i < length; i++) {
	d[i] = vtpci_legacy_read_config_1(sc, off + i)
}

[andrew]

Simplify the for loops

[bryanv]

LGTM