pseries: rework PAPR virtual SCSI

pseries: rework PAPR virtual SCSI

The patch reimplements handling of indirect requests in order to
simplify upcoming live migration support.
- all pointers (except SCSIRequest*) were replaces with integer
indexes and offsets;
- DMA'ed srp_direct_buf kept untouched (ie. BE format);
- vscsi_fetch_desc() is added, now it is the only place where
descriptors are fetched and byteswapped;
- vscsi_req struct fields converted to migration-friendly types;
- many dprintf()'s fixed.

This also removed an unused field 'lun' from the spapr_vscsi device
which is assigned, but never used.  So, remove it.

[David Gibson: removed unused 'lun']
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-id: 1374175984-8930-7-git-send-email-aliguori@us.ibm.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>