conf: avoid looking up network port that doesn't exist

conf: avoid looking up network port that doesn't exist

If the hypervisor driver has not yet created the network port, the
portid field will be "00000000-0000-0000-0000-000000000000".

If a failure occurs during early VM startup, the hypervisor driver may
none the less try to release the network port, resulting in an
undesirable warning:

2019-09-12 13:17:42.349+0000: 16544: error :
virNetworkObjLookupPort:1679 : network port not found: Network port with
UUID 00000000-0000-0000-0000-000000000000 does not exist

By checking if the portid UUID is valid, we can avoid polluting the logs
in this way.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>