lxcapi-snapshot: don't snapshot directory-backed containers

lxcapi-snapshot: don't snapshot directory-backed containers

Instead force a copy clone.  Else if the user makes a change
to the original container, the snapshot will be affected.
The user should first create a snapshot clone, then use
and snapshot that clone while leaving the original container
untouched.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>