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 a1af255f065cc.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
