'change' and related operations did not work when used on guest devices
featuring removable media but no actual tray, because
blk_dev_is_tray_open() always returned false for them and the
blockdev-{insert,remove}-medium commands required it to return true.

Fix this by making blockdev-{insert,remove}-medium work on tray-less
devices. Also, blockdev-{open,close}-tray are now explicitly no-ops when
invoked on such devices, and blk_dev_change_media_cb() is instead
called by blockdev-{insert,remove}-medium (for tray-less devices only).

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-stable <qemu-stable@nongnu.org>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 1454096953-31773-3-git-send-email-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>