KVM: selftests: Convert get-reg-list away from its "VCPU_ID"

Track the vCPU's 'struct kvm_vcpu' object in get-reg-list instead of
hardcoding '0' everywhere.

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