[UPnP] Call found_server callback on GetServers() results

This was not needed before as GetServers() results were always
handled in __init__() -- so no callbacks could yet exist. Now
with asynchronous GetServers() handling this is no longer true.

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
[GenericModel] Add abstract GenericModel._fetch_items()

This seems like a good practice and silences a pychecker warning.

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
[UPnP] Do not make duplicate found_server_callback() calls

Use the same code for GetServers() callback and the FoundServer
handler: GetServers() code path wasn't checking for existing servers,
so duplicates could appear with good timing.

Also mark signal handlers as 'private' methods.

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>