vkernel - Fix problem where cpu-bound user process cannot be interrupted

vkernel - Fix problem where cpu-bound user process cannot be interrupted

* Fix an issue where a cpu-bound user process running in a vkernel cannot
  be interrupted from within the vkernel.

  The problem occurs because the timer interrupt was not marked MPSAFE,
  causing the interrupt thread to hold the MP token which then prevented
  the thread preemption code from letting the timer interrupt thread
  preempt the currently running user process.

* Fixed by marking the timer interrupt and other vkernel interrupt
  handlers as being MPSAFE.

* This is a problem for the vkernel and not for normal kernel.  Normal
  kernels have a doreti function which 'catches' pending flags on
  any attempt to return to userland.

  The vkernel does not, instead relying on the preemption mechanic to
  catch pending flags.

Reported-by: Antonio Huete Jimenez <ahuete.devel@gmail.com>