stubdom: set xen interface version for stubdom apps using xenctrl.h

A stubdom app using xenctrl.h must use the latest interface version of
Xen in order to avoid compatibility issues. Add the related config
item to the stubdom config files where needed.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

stubdom/ioemu-minios.cfg

@@ -1,3 +1,4 @@
 CONFIG_START_NETWORK=n
 CONFIG_QEMU_XS_ARGS=y
 CONFIG_PCIFRONT=y
+XEN_INTERFACE_VERSION=__XEN_LATEST_INTERFACE_VERSION__