machine: Fix replacement of '_' by '-' in machine property names

machine_set_property() replaces '_' by '-' in the property name.
Except it fails to replace an initial '_'.  Screwed up in commit
b0ddb8b.  Reproducer: "-M pc,__foo_bar=true" produces "Property
'._-foo-bar' not found".

Error messages using a mangled name rather than the name the user
actually wrote is user-hostile, but that's a different topic.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

vl.c

@@ -2804,17 +2804,16 @@ static int machine_set_property(void *opaque,
 {
     Object *obj = OBJECT(opaque);
     Error *local_err = NULL;
-    char *c, *qom_name;
+    char *p, *qom_name;
 
     if (strcmp(name, "type") == 0) {
         return 0;
     }
 
     qom_name = g_strdup(name);
-    c = qom_name;
-    while (*c++) {
-        if (*c == '_') {
-            *c = '-';
+    for (p = qom_name; *p; p++) {
+        if (*p == '_') {
+            *p = '-';
         }
     }