libvirt: Fix spurious backing file existence check.

Bug 1195877

The problem lies with the truncation of backing file names at the
first underscore character. On a reboot, nova-compute checks the
backing images exist. Typically, they will do; however, instead
of looking for an "ephemeral_XXX" file, it hunts for "ephemeral".
This won't exist, triggering code that attempts to pull the whole
image again from glance(!)

Lacking credentials, the attempt to fetch the image will fail,
ultimately resulting in the instance going to an ERROR state.

Change-Id: Iabcc7a1ffd248c22a747a1ddc7863d2b386d409a
Merge "libvirt: Fix spurious backing file existence check."