RISC-V: KVM: Use G-stage name for hypervisor page table

The two-stage address translation defined by the RISC-V privileged
specification defines: VS-stage (guest virtual address to guest
physical address) programmed by the Guest OS  and G-stage (guest
physical addree to host physical address) programmed by the
hypervisor.

To align with above terminology, we replace "stage2" with "gstage"
and "Stage2" with "G-stage" name everywhere in KVM RISC-V sources.

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