Fix for https://github.com/Qiskit/qiskit-aer/issues/2258

[aromanro]

Summary

Calling AerState::probability throws an exception with the message "Save amplitude square must be defined on full width of qubits."

Details and comments

Issue: #2258

The fix is having AerState::probability fill the op.qubits with the list of qubits:

  for (uint_t i = 0; i < num_of_qubits_; ++i)
    op.qubits.push_back(i);