xen: Add Xen pvSCSI protocol description

Add the definition of pvSCSI protocol used between the pvSCSI frontend
in a XEN domU and the pvSCSI 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:
- Added comments.
- Adapt to Linux style guide.
- Add support for larger SG-lists by putting them in an own granted
  page.
- Remove stale definitions.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>