Support VmHost#incr_graceful_reboot to gracefully reboot a host

[jeremyevans]

In a graceful reboot, the host is set to draining, and when it no longer has any VMs, it transitions to reboot. After the reboot, it is automatically put back in accepting state.

[fdr]

Added @bsatzger since his project with @0xjunhao is kinda in the same headspace