vfio-ccw: Refactor IRQ handlers

To simplify future expansion.

Signed-off-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20200505122745.53208-6-farman@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>