configure: CONFIG_NO_XEN is duplicated

We already define it in Makefile.target.  But we need to avoid a
curious double negation in order to eliminate it.
Tested-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NEdgar E. Iglesias <edgar.iglesias@gmail.com>

configure

@@ -4299,12 +4299,9 @@ case "$target_arch2" in
       if test "$xen_pci_passthrough" = yes; then
         echo "CONFIG_XEN_PCI_PASSTHROUGH=y" >> "$config_target_mak"
       fi
-    else
-      echo "CONFIG_NO_XEN=y" >> $config_target_mak
     fi
     ;;
   *)
-    echo "CONFIG_NO_XEN=y" >> $config_target_mak
 esac
 case "$target_arch2" in
   arm|i386|x86_64|ppcemb|ppc|ppc64|s390x)

include/hw/xen/xen.h

@@ -25,7 +25,7 @@ extern bool xen_allowed;
 
 static inline bool xen_enabled(void)
 {
-#if defined(CONFIG_XEN_BACKEND) && !defined(CONFIG_NO_XEN)
+#if defined(CONFIG_XEN_BACKEND) && defined(CONFIG_XEN)
     return xen_allowed;
 #else
     return 0;