Stéphane noticed that lxc-snapshot of a dir-backed container
created an overlayfs container.  The expectation is that the
user can continue to modify the original container and later make
a new snapshot, but this doesn't work with the existing behavior -
the overlayfs clone will end up with the modified contents.

So add a 'LXC_CLONE_KEEPBDEVTYPE' flag, which c->snapshot()
passes to c->clone().

Also add a LXC_CLONE_MAYBE_SNAPSHOT.  If this is set and a
backing store does not support snapshotting, then proceed with
a copy clone.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>