conf: parser/formatter/rng for <forward mode='hostdev'>

This patch introduces the new forward mode='hostdev' along with
attribute managed. Includes updates to the network RNG and new xml
parser/formatter code.

Signed-off-by: Shradha Shah <sshah@solarflare.com>

tests/networkxml2xmltest.c

@@ -106,6 +106,8 @@ mymain(void)
     DO_TEST("bandwidth-network");
     DO_TEST("openvswitch-net");
     DO_TEST_FULL("passthrough-pf", VIR_NETWORK_XML_INACTIVE);
+    DO_TEST("hostdev");
+    DO_TEST_FULL("hostdev-pf", VIR_NETWORK_XML_INACTIVE);
 
     return ret==0 ? EXIT_SUCCESS : EXIT_FAILURE;
 }