Add Cloudius copyright and license statement to drivers/*.

A couple of header files were based on Linux's BSD-licensed header files
(e.g., include/uapi/linux/virtio_net.h) so they included the BSD license,
but not any copyright statement, so we can just replace that by our own
statement of the BSD license.

pci::pci_device is redundant, use pci::device.

The pci layer knows about virtio devices and creates them, even though
virtio is a higher layer.

Fix by making virtio_device contain a reference to the pci device, instead
of inheriting from it.  The functionality could probably be moved to
pci_driver.