vhost-scsi: fix k->set_guest_notifiers() NULL dereference

Coverity picked up a copy-paste bug.  In vhost_scsi_start() we check for
!k->set_guest_notifiers and error out.  The check probably got copied
but instead of erroring we actually use the function pointer!

Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Cc: Asias He <asias@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
