xen/grant-table: Force to use v1 of grants.

We have the framework to use v2, but there are no backends that
actually use it. The end result is that on PV we use v2 grants
and on PVHVM v1. The v1 has a capacity of 512 grants per page while
the v2 has 256 grants per page. This means we lose about 50%
capacity - and if we want more than 16 VIFs (each VIF takes
512 grants), then we are hitting the max per guest of 32.

Oracle-bug: 16039922
CC: annie.li@oracle.com
CC: msw@amazon.com
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>