scsi-disk: Fix crash if request is invaild or disk is no medium

scsi-disk: Fix crash if request is invaild or disk is no medium

Qemu will crash with the assertion error that "assert(r->req.aiocb !=
NULL)" in scsi_read_complete if request is invaild or disk is no medium.
The error is below:
qemu-kvm: hw/scsi/scsi_disk.c:299: scsi_read_complete: Assertion
`r->req.aiocb != NULL' failed.

This patch add a funtion scsi_read_complete_noio to fix it.

Signed-off-by: Zhengui Li <lizhengui@huawei.com>
Message-Id: <1551949966-20092-1-git-send-email-lizhengui@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>