virtio: enable VTNET_LEGACY_TX when ALTQ is enabled.

ALTQ only works on network drivers which use if_start (rather than
if_transmit). vtnet uses if_start if built with VTNET_LEGACY_TX. Default
to that the kernel is built with ALTQ enabled, to reduce user surprise.

MFC after:	1 week
Sponsored by:	Rubicon Communications, LLC ("Netgate")

sys/dev/virtio/network/if_vtnetvar.h

@@ -31,6 +31,10 @@
 #ifndef _IF_VTNETVAR_H
 #define _IF_VTNETVAR_H
 
+#ifdef ALTQ
+#define	VTNET_LEGACY_TX
+#endif
+
 struct vtnet_softc;
 
 struct vtnet_statistics {