commit d631b94e
    virtio: change comment in transmit

started clarifying the logic behind queue state management,
but introduced an inaccuracy: TX_BUSY does not cause
a BUG message.

Clean this up some more, explaining the tradeoffs in detail.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>