virtio-scsi: Add real 2-clause BSD license to header

This is analogous to commit a1b383870a made by Rusty Russell to all
the VirtIO headers at the time. This eases the use of the header as
is by other OSes.

Signed-off-by: Bryan Venteicher <bryanv@daemoninthecloset.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>