qemu: Use the 'device_id' property of SCSI disks to avoid regressing

QEMU accidentally exposed the id of -drive (or same value as disk
serial, if provided) in one of the identifiers visible from the guest.

To avoid regression in case when -blockdev will be used we need to
always specify it ourselves.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>