Historically the 'scsi' passthrough feature of virtio-blk-pci
was enabled by default. Libvirt was disabling it due to security
implications outlined in libvirt commit v0.9.9-4-g177db087 if it was
not explicitly requested. In qemu commit v2.4.0-1566-ged65fd1a27 the
default value was changed to disabled in preparation for virtio-1.
Starting from QEMU-5.0 the 'scsi' property was also deprecated. There
replacement for the functionality is to use 'virtio-scsi' for the
purpose. This isn't a direct replacement though.

Add capability named QEMU_CAPS_VIRTIO_BLK_SCSI_DEFAULT_DISABLED which
allows us to stop formatting the 'scsi=' property if it's disabled by
default and not requested so that we don't use deprecated features.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>