Fix AncillaQubits added via .add_bits to be included in .qubits.

[kdk]

Summary

As reported by @Cryoris , QuantumCircuit.add_bits (added in #5486 ) was handling AncillaQubits incorrectly, leading to the following:

>>> qr, ar = QuantumRegister(1), AncillaRegister(1)
>>> circuit = QuantumCircuit(ar, qr)
>>> circuit.x(0)

>>> circuit.draw()

      ┌───┐
a0_0: ┤ X ├
      └───┘
q0_0: ─────

>>> circuit.decompose.draw()

q0_0: ─────────────
      ┌───────────┐
a0_0: ┤ U3(π,0,π) ├
      └───────────┘

Details and comments