Regenerate missing resized backing files

Fixes bug 1185588

The only way to reliably determine whether we are using a legacy
(pre-Grizzly) backing structure is to look at the backing file
reference for the instance/disk itself. Thus, if we determine the
backing file contains an underscore, we use the value following it
as our legacy_backing_size, which is then used to create the
necessary backing file.

Change-Id: I145f71068c972a604e5361ae79d6315286136786
Adds check that the core V3 API is loaded

Adds infrastructure to be able to check that the core V3 API
plugins are present and raise an exception and abort if any are missing. The
list is deliberately hard coded though it is not yet complete and
will be expanded as the core APIs are ported and it is decided
exactly what will be considered to be the core API.

Partially implements blueprint v3-api-extension-framework

Change-Id: Ic2c1d5cd6836ee18819106558880682cf4cda487
Merge "Regenerate missing resized backing files"
Merge "Adds check that the core V3 API is loaded"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp