Fix migrating a launched instance when using LVM image backends.

The instance's backend type (e.g. raw, LVM, qcow2) is preserved
during migration. In other words, if the system is configured to
use LVM backends, then the instance's back will remain LVM after
migration. In contrast, the instance's backend will always end up
as qcow2 after being launched. This different caused issues when
migrating a launched instance because migration would attempt
to keep the configured backend, LVM, for an instance backed by
qcow2.

To fix this, if there is no LVM information associated with the
instance during migration then a qcow2 backend will be used.

Fixes OS-221

Change-Id: I6d84a7bb570222ae658ff726c331e3fee80a24b3