RISC-V: KVM: Rename dis_idx to ext_idx

Make the name of the extension_disabled[] index more general in
order to expand its application.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Anup Patel <anup@brainfault.org>