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
Improve logging of external events on the compute node

We don't log every event we receive on the compute node, which could
allow some to slip through and look like they were never received. This
patch makes sure to debug log each one so we at least know that it made
it from API to compute.

Change-Id: I817248ea8167be21c55fecad3a39cb45b027596c
Related-bug: #1357476
Merge "compute: Add standard constants for hypervisor virt types"
Merge "Improve logging of external events on the compute node"