Add '<nat>' element to '<forward>' network schemas

Commits 905629f4 and 1716e7a6 have added support for specifying
an IPv4 range and a port range to be used by NAT:
<forward mode='nat'>
  <nat>
    <address start='10.20.30.40' end='10.20.30.44'/>
    <port start='60000' end='65432'/>
  </nat>
</forward>

https://bugzilla.redhat.com/show_bug.cgi?id=1004364

tests/networkxml2xmltest.c

@@ -108,6 +108,7 @@ mymain(void)
     DO_TEST("nat-network-dns-srv-record-minimal");
     DO_TEST("nat-network-dns-hosts");
     DO_TEST("nat-network-dns-forward-plain");
+    DO_TEST("nat-network-forward-nat-address");
     DO_TEST("8021Qbh-net");
     DO_TEST("direct-net");
     DO_TEST("host-bridge-net");