bdev: implement btrfs

A copy-clone of btrfs is done by calling the directory backend
clone, as it's simply an rsync.  The snapshot is done using the
btrfs ioctl.

It would be nice if there were a btrfs development library
package with simple functions to do this stuff, but AFAICS
there is not, so I've copied some struct definitions and
ioctl value defines from the btfs-tools source into bdev.c

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>