Re-arrange code setting up ifs/disk loop devices for LXC

Re-arrange code setting up ifs/disk loop devices for LXC

The current code for setting up loop devices to LXC disks first
does a switch() based on the disk format, then looks at the
disk driver name. Reverse this so it first looks at the driver
name, and then the disk format. This is more useful since the
list of supported disk formats depends on what driver is used.

The code for setting loop devices for LXC fs entries also needs
to have the same logic added, now the XML schema supports this.

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