Bug fixes.

- Don't enable the HLT intercept by default. It will be enabled by bhyve(8)
  if required. Prior to this change HLT exiting was always enabled making
  the "-H" option to bhyve(8) meaningless.

- Recognize a VM exit triggered by a non-maskable interrupt. Prior to this
  change the exit would be punted to userspace and the virtual machine would
  terminate.