xen: select grant interface version

Grant v2 will be needed in cases where a frame number in the grant
table can exceed 32 bits. For PV guests this is a host feature, while
for HVM guests this is a guest feature.

So select grant v2 in case frame numbers can be larger than 32 bits
and grant v1 else.

For testing purposes add a way to specify the grant interface version
via a boot parameter.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>