lxcapi_clone: set the right environment variable for mounted fs

If the container is dir-backed, we don't actually mount it (to
support unprivileged use).  So always set the LXC_ROOTFS_MOUNT
to bdev->dest, not to the rootfs path specified in the container
configuration.

This should fix bug http://pad.lv/1253573

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