on copy-clone of btrfs, create a subvolume

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
lxc_clone.c: respect the command line usage of the old script

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
lxc-clone: update documentation

Also refuse to try changing backing stores expect to overlayfs, as
it is not implemented (yet) anyway.

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