Simplify the Storage.S signature

The read and write functions both have signature:

  t -> buf -> offset_sectors -> len_sectors

where
  t : the virtual disk
  buf : a Cstruct wrapping the request payload (for read or write)
  offset_sectors : offset from the beginning of [t] in sectors
  len_sectors : length of request in sectors

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