RISC-V: KVM: Factor-out ONE_REG related code to its own source file

The VCPU ONE_REG interface has grown over time and it will continue
to grow with new ISA extensions and other features. Let us move all
ONE_REG related code to its own source file so that vcpu.c only
focuses only on high-level VCPU functions.

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