block: Leave enforcing tray lock to device models

The device model knows best when to accept the guest's eject command.
No need to detour through the block layer.

bdrv_eject() can't fail anymore.  Make it void.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>