usb: Add Xen pvUSB protocol description

Add the definition of pvUSB protocol used between the pvUSB frontend in
a Xen domU and the pvUSB backend in a Xen driver domain (usually Dom0).

This header was originally provided by Fujitsu for Xen based on Linux
2.6.18.

Changes are:
- adapt to Linux kernel style guide
- use Xen namespace
- add lots of comments
- don't use kernel internal defines

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20211123132048.5335-2-jgross@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>