RISC-V: Make virt header comment title consistent

Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Signed-off-by: Michael Clark <mjc@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>

include/hw/riscv/virt.h

@@ -1,5 +1,5 @@
 /*
- * SiFive VirtIO Board
+ * QEMU RISC-V VirtIO machine interface
  *
  * Copyright (c) 2017 SiFive, Inc.
  *