tests: check-qom-proplist: fix leak

user_creatable_add_opts() returns a reference (the other reference is
for the root parent/child link).

Leak introduced in commit a1af255f.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

tests/check-qom-proplist.c

@@ -428,6 +428,8 @@ static void test_dummy_createcmdl(void)
     g_assert(err == NULL);
     error_free(err);
 
+    object_unref(OBJECT(dobj));
+
     /*
      * cmdline-parsing via qemu_opts_parse() results in a QemuOpts entry
      * corresponding to the Object's ID to be added to the QemuOptsList