virt driver support custom cpu model for guest VCPU

Currently in Nova, what kind of cpu model exposed for guest vcpu is depended
the host configuration. There are two options cpu_mode and cpu_model which
defined cpu model of guest running on specific host. This ways isn't flexible
and didn't utilize the resource effective.

For example, there is new host with new cpu model add to the cluster. But for
support live migration between the new host and old host, the operator have to
set the cpu model to the old one which compatible with other host by
CONF.libvirt.cpu_model. This lose the chance to expose new cpu feature to the
user. But if the operator enable the host expose its new cpu model, some
instances schedule to the new host, but they needn't the new features, and at
same time they lose the chance of live migration.

This blueprint aim to enable user can custom cpu model by flavor/image

This is separated from https://blueprints.launchpad.net/nova/+spec/more-image-properties