xen/guest: prepare hypervisor ops to use alternative calls

xen/guest: prepare hypervisor ops to use alternative calls

Adapt the hypervisor ops framework so it can be used with the
alternative calls framework. So far no hooks are modified to make use
of the alternatives patching, as they are not in any hot path.

No functional change intended.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Wei Liu <wl@xen.org>
Reviewed-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>