compute: Add standard constants for hypervisor virt types

Introduce a nova/compute/hvtype.py module to serve a similar
role to nova/compute/vm_mode.py, by providing constants for known
hypervisor virtualization types. This is primarily used as second
component of entries in the 'supported_instances' host state
property. It is not to be confused with the hypervisor driver
types.

Related-bug: #1348623
Change-Id: I553697992b010179e2bbf25fb8c0d471a15e4b3e
Merge "compute: Add standard constants for hypervisor virt types"