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")