Xen: public/hvm: sync changes of HVM_PARAM_CALLBACK_VIA ABI from Xen

Sync the changes of HVM_PARAM_CALLBACK_VIA ABI introduced by
Xen commit <ca5c54b6ff05> (public/hvm: export the HVM_PARAM_CALLBACK_VIA
ABI in the API).

Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Tested-by: Julien Grall <julien.grall@arm.com>