xen/pvcalls: Add MODULE_LICENSE()

Since commit ba1029c9 ("modpost: detect modules without a
MODULE_LICENSE") modules without said macro will generate

WARNING: modpost: missing MODULE_LICENSE() in <filename>

While at it, also add module description and attribution.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>