virtio: Add virtio over pci transport driver

This adds a transport driver that implements UCLASS_VIRTIO for
virtio over pci, which is commonly used on x86.

It only supports the legacy interface of the pci transport, which
is the default device that QEMU emulates.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>