block: Inquire tray state before tray-moved events

blk_dev_change_media_cb() is called for all potential tray movements;
however, it is possible to request closing the tray but nothing actually
happening (on a floppy disk drive without a medium).

Thus, the actual tray status should be inquired before sending a
tray-moved event (and an event should be sent whenever the status
changed).

Checking @load is now superfluous; it was necessary because it was
possible to change a medium without having explicitly opened the tray
and closed it again (or it might have been possible, at least). This is
no longer possible, though.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
