Create a module for btrfs: lxcbtrfs.{c,h}.

The functions:

	- get_btrfs_subvol_path()
	- btrfs_list_get_path_rootid()
	- is_btrfs_fs()
	- btrfs_detect()
	- btrfs_mount()
	- btrfs_umount()
	- btrfs_subvolume_create()
	- btrfs_same_fs()
	- btrfs_snapshot()
	- btrfs_snapshot_wrapper()
	- btrfs_clonepaths()
	- btrfs_do_destroy_subvol()
	- get_btrfs_tree_idx()
	- my_btrfs_tree *create_my_btrfs_tree()
	- update_tree_node()
	- add_btrfs_tree_node()
	- free_btrfs_tree()
	- do_remove_btrfs_children()
	- btrfs_recursive_destroy()
	- btrfs_try_remove_subvol()
	- btrfs_destroy()
	- btrfs_create()

and the structs:

	- struct mytree_node
	- struct my_btrfs_tree

move from bdev.{c,h} to lxcbtrfs.{c,h}.

Rename the header file

	- lxc-btrfs.h --> lxcbtrfs.h

Adapt Makefile.am to include lxcbtrfs.{c,h} and remove lxc-btrfs.h.
Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>