Now that drive_del acquires the AioContext we can safely allow deleting
the drive. As with non-dataplane mode, all I/Os submitted by the guest
after drive_del will return EIO.
This patch makes hot unplug work with virtio-blk dataplane. Previously
drive_del reported an error because the device was busy.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>