xen: Clean up build system

Introduce CONFIG_XEN_BACKEND so that this new config solely controls the
target-independent backend build and CONFIG_XEN can focus on per-target
building.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
diff --git a/configure b/configure
index 88159ac..e57efb1 100755
--- a/configure
+++ b/configure
@@ -2850,7 +2850,7 @@
   echo "BLUEZ_CFLAGS=$bluez_cflags" >> $config_host_mak
 fi
 if test "$xen" = "yes" ; then
-  echo "CONFIG_XEN=y" >> $config_host_mak
+  echo "CONFIG_XEN_BACKEND=y" >> $config_host_mak
   echo "CONFIG_XEN_CTRL_INTERFACE_VERSION=$xen_ctrl_version" >> $config_host_mak
 fi
 if test "$io_thread" = "yes" ; then