qemu-config: Accept empty option values

Currently it is impossible to set an option in a config file to an empty
string, because the parser matches only lines containing non-empty
strings between double-quotes.

As sscanf() "[" conversion specifier only matches non-empty strings, add
a special case for empty strings.

Reviewed-by: Eric Blake <eblake@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

util/qemu-config.c

@@ -413,7 +413,9 @@ int qemu_config_parse(FILE *fp, QemuOptsList **lists, const char *fname)
             opts = qemu_opts_create(list, NULL, 0, &error_abort);
             continue;
         }
-        if (sscanf(line, " %63s = \"%1023[^\"]\"", arg, value) == 2) {
+        value[0] = '\0';
+        if (sscanf(line, " %63s = \"%1023[^\"]\"", arg, value) == 2 ||
+            sscanf(line, " %63s = \"\"", arg) == 1) {
             /* arg = value */
             if (opts == NULL) {
                 error_report("no group defined");