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.

The designer of kvmclock wrote it in a way so as to be ABI-compatible
with xen's pvclock. We can reuse the same structures, then, so let's do it.