KVM: Add kvm_vcpu_get_idx to get vcpu index in kvm->vcpus

There are occasional needs to use the index of vcpu in the kvm->vcpus
array to map something related to a VCPU.  For example, unlike the
vcpu->vcpu_id, the vcpu index is guaranteed to not be sparse across all
vcpus which is useful when allocating a memory area for each vcpu.

Signed-off-by: Christoffer Dall <cdall@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>