XaccQuantum: Add setter for accelerator/shots

In my experience, it has been easier to treat XaccQuantum as a singleton
to avoid issues with XACC initializing itself twice.

It is probably cleaner to pass (accelerator, shots) via qiree::Executor,
but this solves my immediate implementaiton problem in the Qwerty
compiler/runtime.

src/qirxacc/XaccQuantum.cc

@@ -51,9 +51,7 @@ XaccQuantum::XaccQuantum(std::ostream& os,
     xacc::setIsPyApi();
 
     // Create accelerator
-    accelerator_ = xacc::getAccelerator(accel_name);
-    QIREE_VALIDATE(accelerator_, << "failed to create accelerator");
-    accelerator_->updateConfiguration({{"shots", static_cast<int>(shots)}});
+    set_accelerator_and_shots(accel_name, shots);
     // TODO: bit order is accelerator-dependent?
     endian_ = Endianness::little;
 
@@ -76,6 +74,17 @@ XaccQuantum::~XaccQuantum()
     xacc::Finalize();
 }
 
+//---------------------------------------------------------------------------//
+/*!
+ * Update the XACC accelerator and shot count.
+ */
+void XaccQuantum::set_accelerator_and_shots(
+    std::string const& accel_name, size_type shots) {
+    accelerator_ = xacc::getAccelerator(accel_name);
+    QIREE_VALIDATE(accelerator_, << "failed to create accelerator");
+    accelerator_->updateConfiguration({{"shots", static_cast<int>(shots)}});
+}
+
 //---------------------------------------------------------------------------//
 /*!
  * Prepare to build a quantum circuit for an entry point.

src/qirxacc/XaccQuantum.hh

@@ -58,6 +58,13 @@ class XaccQuantum final : virtual public QuantumNotImpl
     size_type num_qubits() const { return num_qubits_; }
     //!@}
 
+    //!@{
+    //! \name Mutators
+    // Update the XACC accelerator and shot count
+    void set_accelerator_and_shots(
+        std::string const& accel_name, size_type shots);
+    //!@}
+
     //!@{
     //! \name Quantum interface
     // Prepare to build a quantum circuit for an entry point