RISC-V: KVM: Implement trap & emulate for hpmcounters

As the KVM guests only see the virtual PMU counters, all hpmcounter
access should trap and KVM emulates the read access on behalf of guests.

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