RISC-V: KVM: Disable all hpmcounter access for VS/VU mode

Any guest must not get access to any hpmcounter including cycle/instret
without any checks. We achieve that by disabling all the bits except TM
bit in hcounteren.

However, instret and cycle access for guest user space can be enabled
upon explicit request (via ONE REG) or on first trap from VU mode
to maintain ABI requirement in the future. This patch doesn't support
that as ONE REG interface is not settled yet.

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