[Feature] Migrating of QEMU machines

Currently, Nomad does not handle QEMU allocation migration properly. According to @diptanu

> Yeah we simply shutdown the process. So no we don't shutdown the VM properly.

My use case specifically would be a Windows VMs and migrating those allocations between nodes. If I'm running a SQL Server database or IIS web server as an example, I wouldn't the VM process to just be killed and restarted on another node. I would almost prefer some form of "live migration" across hosts. The reason for this specifically is because most Windows software doesn't handle shutdowns very well or at all so just rapidly killing the process could have adverse effects. Another scenario would be when issuing a node-drain on a host.

The issue names no files, tests, or entry points. Clarify the supported QEMU migration behavior, especially for Windows VMs and node drains, then define tests that demonstrate the VM is migrated without being abruptly killed.

Written by the indexing model from the issue text.