kernel - Fix a system lockup with vmm

kernel - Fix a system lockup with vmm

* Fix an issue where vkernel_lwp_exit() was improperly trying to kfree()
  the vklp->ve pointer for the guest-thread case.  This field holds a
  user-supplied address in that case, not a kernel structure.

* Yield the cpu more aggressively in the VMM_GUEST_RUN loop.  We were
  testing for pending interrupts but we were not calling lwkt_switch()

* Do not exit the vkernel on a call or jump to address 0.  This debugging
  code should have been removed and wasn't.  A user process running under
  the vkernel could cause the vkernel itself to exit.

* Numerous syntactical cleanups.

Reported-by: tuxillo