hw/core: Allow setting 'virtio-blk-device.scsi' property on OSX host

hw/core: Allow setting 'virtio-blk-device.scsi' property on OSX host

Commit ed65fd1a2750 ("virtio-blk: switch off scsi-passthrough by
default") changed the default value of the 'scsi' property of
virtio-blk, which is only available on Linux hosts. It also added
an unconditional compat entry for 2.4 or earlier machines.

Trying to set this property on a pre-2.5 machine on OSX, we get:

   Unexpected error in object_property_find() at qom/object.c:1201:
   qemu-system-x86_64: -device virtio-blk-pci,id=scsi0,drive=drive0: can't apply global virtio-blk-device.scsi=true: Property '.scsi' not found

Fix this error by marking the property optional.

Fixes: ed65fd1a27 ("virtio-blk: switch off scsi-passthrough by default")
Suggested-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-id: 20200207001404.1739-1-philmd@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>