configure: add missing --disable-modules option

According to ./configure all options should have both --enable-foo and
--disable-foo:

  # Always add --enable-foo and --disable-foo command line args.
  # Distributions want to ensure that several features are compiled in, and it
  # is impossible without a --enable-foo that exits if a feature is not found.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1446473183-24250-1-git-send-email-stefanha@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

configure

@@ -791,6 +791,9 @@ for opt do
   --enable-modules)
       modules="yes"
   ;;
+  --disable-modules)
+      modules="no"
+  ;;
   --cpu=*)
   ;;
   --target-list=*) target_list="$optarg"