megasas: always store SCSIRequest* into MegasasCmd

This ensures that the request is unref'ed properly, and avoids a
segmentation fault in the new qtest testcase that is added.
This is CVE-2017-9503.

Reported-by: Zhangyanyu <zyy4013@stu.ouc.edu.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
