qbus: Initialize in standard way

Move code to an initfn and finalizer.
Replace do_qbus_create_inplace() with qbus_realize().

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
