RISC-V: KVM: Add extensible system instruction emulation framework

We will be emulating more system instructions in near future with
upcoming AIA, PMU, Nested and other virtualization features.

To accommodate above, we add an extensible system instruction emulation
framework in vcpu_insn.c.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>