virtio-scsi: fix with -M pc-i440fx-2.0

Right now starting a machine with virtio-scsi and a <= 2.0 machine type
fails with:

    qemu-system-x86_64: -device virtio-scsi-pci: Property .any_layout not found

This is because the any_layout bit was actually never set after
virtio-scsi was changed to support arbitrary layout for virtio buffers.

(This was just a cleanup and a preparation for virtio 1.0; no guest
actually checks the bit, but the new request parsing algorithms are
tested even with old guest).

Reported-by: David Gilbert <dgilbert@redhat.com>
Reviewed-by: David Gilbert <dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>