s390: Move IPL code into a separate device

s390: Move IPL code into a separate device

Lets move the code to setup IPL for external kernel
or via the zipl rom into a separate file. This allows to

- define a reboot handler, setting up the PSW appropriately
- enhance the boot code to IPL disks that contain a bootmap that
  was created with zipl under LPAR or z/VM (future patch)
- reuse that code for several machines (e.g. virtio-ccw and virtio-s390)
- allow different machines to provide different defaults

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
[agraf: symbolify initial psw, adjust header file location, fix for QOM]
Signed-off-by: Alexander Graf <agraf@suse.de>