Add XEN pvSCSI support

[j***@suse.com]

This series adds XEN pvSCSI support. With pvSCSI it is possible to use physical
SCSI devices from a XEN domain.

The support consists of a backend in the privileged Domain-0 doing the real
I/O and a frontend in the unprivileged domU passing I/O-requests to the backend.

The code is taken (and adapted) from the original pvSCSI implementation done
for Linux 2.6 in 2008 by Fujitsu.

[PATCH V2 1/4] Add XEN pvSCSI protocol description
[PATCH V2 2/4] Introduce xen-scsifront module
[PATCH V2 3/4] Introduce XEN scsiback module
[PATCH V2 4/4] add xen pvscsi maintainer

Changes in V4:
- Re-add define for VSCSIIF_ACT_SCSI_SG_PRESET to vscsiif.h to indicate this
action value should not be used in future enhancements

Changes in V3:
- added some comments to the protocol header file
- removed the CDB emulation from xen-scsiback, handled by core target
infrastructure
- several changes in xen-scsifront after comments from Christoph Hellwig

Changes in V2:
- use core target infrastructure by backend instead of pure SCSI passthrough
- add support for larger SG lists by putting them in grant page(s)
- add command abort capability