block: Revert entanglement of bdrv_is_inserted() with tray status

Commit 4be9762a changed bdrv_is_inserted() to fail when the tray is
open.  Unfortunately, there are two different kinds of users, with
conflicting needs.

1. Device models using bdrv_eject(), currently ide-cd and scsi-cd.
They expect bdrv_is_inserted() to reflect the tray status.  Commit
4be9762a makes them happy.

2. Code that wants to know whether a BlockDriverState has media, such
as find_image_format(), bdrv_flush_all().  Commit 4be9762a makes them
unhappy.  In particular, it breaks flush on VM stop for media ejected
by the guest.

Revert the change to bdrv_is_inserted().  Check the tray status in the
device models instead.

Note on IDE: Since only ATAPI devices have a tray, and they don't
accept ATA commands since the recent commit "ide: Reject ATA commands
specific to drive kinds", checking in atapi.c suffices.

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