RISC-V: KVM: Fix incorrect KVM_MAX_VCPUS value

The KVM_MAX_VCPUS value is supposed to be aligned with number of
VMID bits in the hgatp CSR but the current KVM_MAX_VCPUS value
is aligned with number of ASID bits in the satp CSR.

Fixes: 99cdc6c1 ("RISC-V: Add initial skeletal KVM support")
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>