vkernel - Fix deadlocks with cothread locks

vkernel - Fix deadlocks with cothread locks

* Callbacks into the main kernel are not allowed when holding a cothread
  lock as this could cause a deadlock between cpus.  If the callback into
  the kernel blocks the cothread lock, being a pthreads lock,
  remains locked.

* Refactor the network and disk pipeline to hold the cothread lock for
  a much shorter period of time, allowing data to be pipelined without
  any stall conditions.

* For vknet if the tx mbuf return fifo is full we wait until it isn't.