Stacked mounts don't work as expected

[matthewsucar]

Due to the lack of an intermediate representation (pre-chroot filesystem), stacking mounts doesn't work due to the check_path() check. check_path operates on the host path, where the intermediate mount is in the image's tree (inside the namespace)

For example:

from: /tmp
to:/picnic,
from:/picnic
to:/picnic

(assuming /picnic doesn't exist outside the namespace).

This could be solved by presenting the admin with an option to disable the sanity checks. Otherwise, we need to pass the in-image path to check_path() only when we are stacking mounts

This is really only necessary if we assume that the image can't/shouldn't be prepared in advance

[naterini]

Need to verify this is still broken after 1396027.