KVM: riscv: selftests: Add a test for PMU snapshot functionality

KVM: riscv: selftests: Add a test for PMU snapshot functionality

Verify PMU snapshot functionality by setting up the shared memory
correctly and reading the counter values from the shared memory
instead of the CSR.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20240420151741.962500-23-atishp@rivosinc.com
Signed-off-by: Anup Patel <anup@brainfault.org>