pupnp: fix hanging of UpnpDownloadXmlDoc in PUPnP

Sadly, this method is async, and can cause really long delay in
shutdown, but at least, avoid to call UPnPFinish before the end
of the download (else this method never returns)

GitLab: #8
Change-Id: I80df3da9b884b93a331aae738eae9a27be898fe2