With the change back to using a directory backed container by default,
the path /var/lib/machines is no longer assumed to be running BTRFS.
This change checks the `container_tech` option and if it's nspawn sets
the backend store to BTRFS, otherwise the normal
`lxc_container_backing_store` option will be used to determine the data
disk format.
Change-Id: I8ad207d4d2b09f15cdf95b5c6c7fc8fa22449f8c
Signed-off-by: Kevin Carter <kevin.carter@rackspace.com>