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>

Author: jbeulich

drivers/xen/Kconfig

@@ -214,7 +214,7 @@ config XEN_PVCALLS_FRONTEND
 	  implements them.
 
 config XEN_PVCALLS_BACKEND
-	bool "XEN PV Calls backend driver"
+	tristate "XEN PV Calls backend driver"
 	depends on INET && XEN && XEN_BACKEND
 	help
 	  Experimental backend for the Xen PV Calls protocol