Since specifying lxc.rootfs.backend can lead to performance improvements we
always set it during container creation.

This also fixes a bug. do_bdev_create() tried to be smart and retrieve the lower
directory when bdev->type == overlayfs or aufs thereby cutting the path.
However, this operation is done in ovl_mount() and aufs_mount() and both
functions need the full src path for this. The bug didn't show before because
when creating a overlayfs container with e.g.:

	lxc-create -n c -t busybox -B overlayfs

still left bdev->type set to dir and so the code for retrieving the lower dir in
do_bdev_create() was skipped. But by setting lxc.rootfs.backend on container
creation we force bdev->type to be set to e.g. overlayfs and the code gets
executed thereby exposing the bug.
Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>