s390x/sclp: add device to the sysbus in sclp_realize

The init of a device should have no side effects. Therefore move
registering of the event facility into the realize function, so
multiple instances of the SCLP device can be created e.g. for
introspection.

Add some more detail as to why we have to add it to the sysbus
at all.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>