xen/pvcalls: backend can be a module

It's not clear to me why only the frontend has been tristate. Switch the
backend to be, too.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>

Link: https://lore.kernel.org/r/54a6070c-92bb-36a3-2fc0-de9ccca438c5@suse.com


Signed-off-by: Juergen Gross <jgross@suse.com>