hw/scsi-bus.c: Fix use of uninitialised variable

Don't use req before it has been initialised in scsi_req_new().
This fixes a compile failure due to gcc complaining about this.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
