RISC-V: KVM: Save/restore SCOUNTEREN in C source

RISC-V: KVM: Save/restore SCOUNTEREN in C source

The SCOUNTEREN CSR need not be saved/restored in the low-level
__kvm_riscv_switch_to() function hence move the SCOUNTEREN CSR
save/restore to the kvm_riscv_vcpu_swap_in_guest_state() and
kvm_riscv_vcpu_swap_in_host_state() functions in C sources.

Also, re-arrange the CSR save/restore and related GPR usage in
the low-level __kvm_riscv_switch_to() low-level function.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20241020194734.58686-4-apatel@ventanamicro.com
Signed-off-by: Anup Patel <anup@brainfault.org>