update pvSCSI protocol description

update pvSCSI protocol description

Update the protocol description of the pvSCSI framework used to pass through
SCSI devices to a guest (pv or hvm).

The main changes are:
- added comments
- add support for larger SG-lists by putting them in an own granted page
- deprecate VSCSIIF_ACT_SCSI_SG_PRESET action with related structures
- add ref_rqid for action VSCSIIF_ACT_SCSI_ABORT to be able to specify the
  request to abort
- deprecate timeout_per_command as this really should be handled by the
  backend in case of default settings or by the guest domain by aborting a
  long running command

This update is related to the rework of the pvSCSI backend and frontend drivers
in the Linux kernel. This interface version is included in that rework, too.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>