API support for container snapshots (v2)

API support for container snapshots (v2)

The api allows for creating, listing, and restoring of container
snapshots.  Snapshots are created as snapshot clones of the
original container - i.e. btrfs and lvm will be done as snapshot,
a directory-backed container will have overlayfs snapshots.  A
restore is a copy-clone, using the same backing store as the
original container had.

Changelog:

 . remove lxcapi_snap_open, which wasn't defined anyway.
 . rename get_comment to get_commentpath
 . if no newname is specified at restore, use c->name (as we meant to)
   rather than segving.
 . when choosing a snapshot index, use the correct path to check for.

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