Regenerate missing resized backing files

Fixes bug 1185588 (for stable/grizzly)

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
Cherry-Pick: 6e768363450774cfee90e41aa5c40af780d3e04a
Merge "Regenerate missing resized backing files" into stable/grizzly