xen: mapcache: Split mapcache_grants by ro and rw

xen: mapcache: Split mapcache_grants by ro and rw

Today, we don't track write-abiliy in the cache, if a user
requests a readable mapping followed by a writeable mapping
on the same page, the second lookup will incorrectly hit
the readable entry.

Split mapcache_grants by ro and rw access. Grants will now
have separate ways in the cache depending on writeability.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@amd.com>