vnc: fix unfinalized tlscreds for VncDisplay

In vnc_display_open(), if tls-creds is enabled, do object_ref(object
ref 1->2) for tls-creds. While in vnc_display_close(), object_unparent
sets object ref to 1(2->1) and  unparent the object for root.
Problem:
1. the object can not be found from the objects_root, while the object
is not finalized.
2. the qemu_opts of tls-creds(id: creds0) is not deleted, so new tls
object with the same id(creds0) can not be delete & add.

Signed-off-by: Zihao Chang <changzihao1@huawei.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210111131911.805-1-changzihao1@huawei.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>