conf: parse/format <port isolated='yes|no'/>

This is a very simple thing to parse and format, but needs to be done
in 4 places, so two trivial utility functions have been made that can
be called from all the higher level parser/formatters:

  <domain><interface>
  <domain><interface><actual> (only in domain status)
  <network>
  <networkport>

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

tests/networkxml2xmltest.c

@@ -160,6 +160,7 @@ mymain(void)
     DO_TEST("metadata");
     DO_TEST("set-mtu");
     DO_TEST("dnsmasq-options");
+    DO_TEST("isolated-ports");
 
     return ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
 }