Do not prematurely close loop devices in LXC controller

Do not prematurely close loop devices in LXC controller

The LXC controller is closing loop devices as soon as the
container has started. This is fine if the loop device
was setup as a mounted filesystem, but if we're just passing
through the loop device as a disk, nothing else is keeping
it open. Thus we must keep the loop device FDs open for as
long the libvirt_lxc process is running.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>