RISC-V: Make virt board description match spike

This makes 'qemu-system-riscv64 -machine help' output more tidy
and consistent.

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

hw/riscv/virt.c

@@ -407,7 +407,7 @@ static const TypeInfo riscv_virt_board_device = {
 
 static void riscv_virt_board_machine_init(MachineClass *mc)
 {
-    mc->desc = "RISC-V VirtIO Board (Privileged spec v1.10)";
+    mc->desc = "RISC-V VirtIO Board (Privileged ISA v1.10)";
     mc->init = riscv_virt_board_init;
     mc->max_cpus = 8; /* hardcoded limit in BBL */
 }