- xl code writes "dir" and "subdirs". It puts permissions on "dirs" first
  and subdirs are assumed to inherit those permissions.
- Two new flags inherit_read and inherit_write were introduced so we could
  inherit capabilities. Once the "dirs" get permissions corresponding inherit
  flag is set. And all the "subdirs" underneath is granted respective caps.
- At this point, default capabilities is not fully implemented. Its side effects
  haven't been observed so far.
- Also, in order to grant capabilities to domains that stamped permissions on
  the "dir", a "grant_flag" is introduced in caps_to_strings. caps_to_strings with
  "grant_flag" set returns the domids that needs to be granted caps.
- Note, during experimenting, I observed that a garbage domid could result in
  crashing xen. This has been fixed now.