RISC-V: KVM: Add ONE_REG interface for mvendorid, marchid, and mimpid

We add ONE_REG interface for VCPU mvendorid, marchid, and mimpid
so that KVM user-space can change this details to support migration
across heterogeneous hosts.

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