lxc-ubuntu: fix btrfs when rootfs == realrootfs

Fix btrfs support when lxc-create does not bind-mount the rootfs.

Signed-off-by: José Martínez <xosemp@gmail.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
lxc-ubuntu: update coding style

Signed-off-by: José Martínez <xosemp@gmail.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>