Fix regression in option parsing

Commit ec229bbe7 broke invocation without a specific -hda.  IOW, qemu foo.img.
The lack of an optind update caused an infinite loop.
Reported-by: NAmit Shah <amit.shah@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

vl.c

@@ -4819,6 +4819,7 @@ int main(int argc, char **argv, char **envp)
     while (optind < argc) {
         if (argv[optind][0] != '-') {
             /* disk image */
+            optind++;
             continue;
         } else {
             const QEMUOption *popt;