Track eject request state in PDO

Tracking the state of the eject request in the frontend was sub-optimal
as:

a) The lifetime of the frontend is not quite the same as the PDO.
b) It was possible for the FDO scan to race with eject and make it
   look like the device had just disappeared.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>