configure: Fix warnings in VDE library probe

Fix compile warnings in the VDE library probe ("passing argument 1 of
'vde_open_real' discards 'const' qualifier from pointer target type",
ditto argument 2).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

configure

@@ -1822,7 +1822,8 @@ if test "$vde" != "no" ; then
 int main(void)
 {
     struct vde_open_args a = {0, 0, 0};
-    vde_open("", "", &a);
+    char s[] = "";
+    vde_open(s, s, &a);
     return 0;
 }
 EOF