MFC r268481:

  Rework when the Tx queue completion interrupt is enabled

  The Tx interrupt is now kept disabled in the common case, only
  enabled when the number of free descriptors in the queue falls
  below a threshold. Transmitted frames are cleared from the VQ
  before subsequent transmit, or in the watchdog timer.

  This was a very big performance improvement for an experimental
  Netmap bhyve backend.