MFC 263780,264516,265062,265101,265203,265364:

MFC 263780,264516,265062,265101,265203,265364:
Add an ioctl to suspend a virtual machine (VM_SUSPEND).

Add logic in the HLT exit handler to detect if the guest has put all vcpus
to sleep permanently by executing a HLT with interrupts disabled.

When this condition is detected the guest with be suspended with a reason of
VM_SUSPEND_HALT and the bhyve(8) process will exit.

This logic can be disabled via the tunable 'hw.vmm.halt_detection'.

git-svn-id: svn://svn.freebsd.org/base/stable/10@268935 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f