dissasembler does not handle multi-control gates correctly

[ColemanCollins]

Information

• Qiskit Terra version: 0.17
• Python version: 3.9.2
• Operating system: macOS 11.2.3 (Big Sur)

What is the current behavior?

When attempting to use assembler.disassemble on a Qobj with an mcx gate, the disassembler throws an error.

Steps to reproduce the problem

from qiskit import assembler, compiler
qc = QuantumCircuit(5,5)
qc.mcx([0,1,2,3], 4)
qobj = compiler.assemble(qc)
assembler.disassemble(qobj)

produces

~/anaconda3/envs/qiskit_env/lib/python3.9/site-packages/qiskit/assembler/disassemble.py in _experiments_to_circuits(qobj)
    128                     _inst = instr_method(*params, qubits, clbits)
    129                 else:
--> 130                     _inst = instr_method(*params, *qubits, *clbits)
    131             elif name == 'bfunc':
    132                 conditional['value'] = int(i.val, 16)

TypeError: mcx() takes from 3 to 5 positional arguments but 6 were given

a shorter version that doesn't break the argument number also hints at what's being passed being incorrect in some way:
(same input as above except qc.mcx([0,1,2],3))

~/anaconda3/envs/qiskit_env/lib/python3.9/site-packages/qiskit/circuit/quantumcircuit.py in mcx(self, control_qubits, target_qubit, ancilla_qubits, mode)
   2536         """
   2537         from .library.standard_gates.x import MCXGrayCode, MCXRecursive, MCXVChain
-> 2538         num_ctrl_qubits = len(control_qubits)
   2539 
   2540         available_implementations = {

TypeError: object of type 'Qubit' has no len()

What is the expected behavior?

correct disassembly into a circuit

Suggested solutions

I think what's happening here is that the signature for quantumcircuit.mcx expects a list of controls, followed by a target; mcx(control_qubits, target_qubit, ...) The qobj representation of an mcx gate is just a list of qubits, with (I think) the last being the target; L130 is unzipping the entire qubit list into the method, which does not match the signature.

granted, this might eventually be moot given #5897, but it looks like there needs to be another case in the disassembler for this gate and any like it that expect a list, or else a change in the gate def to accommodate this case.

[enavarro51]

@ColemanCollins If you're not working on this, I think I have at least a rudimentary solution. It appears that 4 QuantumCircuit methods, mcx, mct, mcp, and mcu1 use the control_qubits list. (There may be others if we look through all the internally created gates, but I think fixing these will handle the immediate problem). It's easy to treat these as a special case in the disassembler.

If you'd like, I can submit a PR to fix this.

[ColemanCollins]

@enavarro51 feel free! no feeling of ownership here and I wasn't planning on getting to it until the weekend

[enavarro51]

Thanks. PR at #6291.