Add virt driver capabilities definition

Going forward with the early stages of no-db-compute, we will
consistently hit cases where we need to eject some database usage
from one virt driver up a few layers. Since those are not commonly
used by all, some way of determining (and the higher layer) the
properties of the virt driver being used will be useful.

We have discussed using something like a mixin interface class
and multiple inheritance. We have discussed tricks like detecting
if the manage_image_cache() method has been overridden. We have
discussed trying it once and catching NotImplementedError and then
never trying again. Aside from the first, none of these are
unsneaky enough to bite us later.

This approach will provide us a way to declare such properties
succinctly in the compute driver to help the higher layers know
what we want them to do on our behalf.

Change-Id: I74dea9322a5b4688319ebf5d9afe416e93401c58

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