[virtio] Add virtio 1.0 PCI support

[virtio] Add virtio 1.0 PCI support

This commit adds support for driving virtio 1.0 PCI devices.  In
addition to various helpers, a number of vpm_ functions are introduced
to be used instead of their legacy vp_ counterparts when accessing
virtio 1.0 (aka modern) devices.

Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>