By using a loopback device, disks backed by plain files can
be made available to LXC containers. We make no attempt to
auto-detect format if <driver type="raw"/> is not set,
instead we unconditionally treat that as meaning raw. This
is to avoid the security issues inherent with format
auto-detection
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>