block: clarify error message for qmp-eject

block: clarify error message for qmp-eject

If you use HMP's eject but the CDROM tray is locked, you may get a
confusing error message informing you that the "tray isn't open."

As this is the point of eject, we can do a little better and help
clarify that the tray was locked and that it (might) open up later,
so try again.

It's not ideal, but it makes the semantics of the (legacy) eject
command more understandable to end users when they try to use it.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>