KVM: selftests: Convert kvm_binary_stats_test away from vCPU IDs

Track vCPUs by their 'struct kvm_vcpu' object in kvm_binary_stats_test,
not by their ID.  The per-vCPU helpers will soon take a vCPU instead of a
VM+vcpu_id pair.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>