qdev/qbus: remove failover specific code

qdev/qbus: remove failover specific code

Commit f3a850565693 ("qdev/qbus: add hidden device support") has
introduced a generic way to hide a device but it has modified
qdev_device_add() to check a specific option of the failover device,
"failover_pair_id", before calling the generic mechanism.

It's not needed (and not generic) to do that in qdev_device_add() because
this is also checked by the failover_hide_primary_device() function that
uses the generic mechanism to hide the device.

Cc: Jens Freimann <jfreimann@redhat.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20211019071532.682717-3-lvivier@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>