qobject: Clean up around qtype_code

QTYPE_NONE is a sentinel value.  No QObject has this type code.
Document it properly.

Fix dump_qobject() to abort() on QTYPE_NONE, just like for any other
invalid type code.

Fix to_json() to abort() on all invalid type codes, not just
QTYPE_MAX.

Clean up Property member qtype's type: it's a qtype_code.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

include/hw/qdev-core.h

@@ -226,7 +226,7 @@ struct Property {
     PropertyInfo *info;
     int          offset;
     uint8_t      bitnr;
-    uint8_t      qtype;
+    qtype_code   qtype;
     int64_t      defval;
     int          arrayoffset;
     PropertyInfo *arrayinfo;