xen/scsiback: use new command result macros

Instead of using the kernel's values for the result of PV scsi
operations use the values of the interface definition.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20220428075323.12853-3-jgross@suse.com


Signed-off-by: Juergen Gross <jgross@suse.com>