Correct typos in the storage interface's phantom types

Correct typos in the storage interface's phantom types

A connection is either read/write (type rw = [`R|`W]) or read/only
(type ro = [`R]). A function which requires the ability to write
needs to take a parameter [> `W] rather than [`W] (which would
correspond to a write-only connection).

We can now use a read/write connection to call:
  Pool.set_autostart
  Volume.create_xml
  Volume.delete

Signed-off-by: David Scott <dave.scott@eu.citrix.com>