virtio-scsi: fix use-after-free of VirtIOSCSIReq

scsi_req_continue can complete the request and cause the VirtIOSCSIReq
to be freed.  Fetch req->sreq just once to avoid the bug.

Reported-by: Richard Jones <rjones@redhat.com>
Tested-by: Richard Jones <rjones@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
