libvirt: Allow specification of default machine type

The libvirt driver currently does not set the machine type for a KVM
guest by default. When not specified, libvirt will use the newest one
it knows about. Unfortunately, that can result in live migrations
failing if your environment is using different versions of the host OS
on compute nodes as the destination node may not be able to support the
machine type used when the VM was originally started.

A simple solution to this is to provide a new option which allows you
to specify the default machine type on a per compute node basis
(nova.conf option). By using this option, you can ensure that VMs are
started with a machine type that will allow it to be live migrated to
other nodes in the deployment.

This patch implements that solution by adding the hw_machine_type
option to the [libvirt] group of nova.conf.

DocImpact
Closes-bug: #1331170

Change-Id: I223c70c729315b6ffc01eb293fe70553ef827162
Merge "libvirt: Allow specification of default machine type"