fix cdrom eject, media change, stubdom fallback case

fix cdrom eject, media change, stubdom fallback case

this patch fixes few cdrom handling issues:

- when a cdrom medium event occurs and the new filename is NULL,
  we should eject the emulated cdrom instead of just ignoring the event;

- when a cdrom medium event occurs and we are in a stubdom, we have to
  pass the xenstore path of the frontend to bdrv_open instead of the
  filename;

- when we try and fail to open a cdrom (or disk) in a stubdom we should
  not fall back to the usual "non-stubdom" code path because is useless
  and wrong.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>