define a VIR_DEFINE_AUTOPTR_FUNC() to autofree virNetworkPortDefs, and
convert all uses of virNetworkPortDefPtr that are appropriate to use
it.

This coincidentally fixes multiple potential memory leaks (in failure
cases) in networkPortCreateXML()
Signed-off-by: NLaine Stump <laine@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>