configure: Assure printing "yes" or "no" for VirtFS support

When auto-detecting VirtFS support, virtfs="". Set it to "no" after
checking whether it was explicitly requested through --enable-virtfs.
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

configure

@@ -2902,6 +2902,7 @@ if test "$softmmu" = yes ; then
       if test "$virtfs" = yes; then
         feature_not_found "virtfs"
       fi
+      virtfs=no
     fi
   fi
   if [ "$linux" = "yes" -o "$bsd" = "yes" -o "$solaris" = "yes" ] ; then