virtio-ccw: move virtio_ccw_stop_ioeventfd to virtio_ccw_busdev_unplug

virtio-ccw: move virtio_ccw_stop_ioeventfd to virtio_ccw_busdev_unplug

Similar to the PCI bug that prompted these patches, virtio-ccw will
segfault after the reworking of hotplug/hot-unplug.  Prepare for
this by moving virtio_ccw_stop_ioeventfd to before the freeing
of the proxy device.

A better place for this could be the device_unplugged callback
for the virtio-ccw bus.  However, we do not yet have a callback
that works: this patch avoids the problem while leaving the tree
bisectable.

Cc: qemu-stable@nongnu.org
Reported-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Suggested-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Acked-by: Andreas Faerber <afaerber@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>