vfio-ccw: Refactor the unregister of the async regions

This is mostly for the purposes of a later patch, since
we'll need to do the same thing later.

While we are at it, move the resulting function call to ahead
of the unregistering of the IOMMU notifier, so that it's done
in the reverse order of how it was created.

Signed-off-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20200505122745.53208-4-farman@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>