fix(instances): a new VM's CPU is a mode or a model the host offers, checked by libvirt

- A named CPU model is written without check='none' and with
  fallback='forbid': libvirt checks before the start that the host runs
  it, and it is not swapped for another model.
- Mode maximum is written as that mode; it was written as a model named
  maximum.
- Model names up to 64 characters: the -noTSX and -IBRS models, the ones a
  mixed set of hosts needs, were refused at 20.
- The mode or model is checked against the host's capabilities when the VM
  is defined, from the page and the API alike; no CPU element (QEMU's
  oldest default CPU) is no longer offered.
- The default CPU mode setting offers the modes the new VM dialog has;
  no-model and custom, which matched none of them, become host-model.

instances/templates/create_instance_w2.html

@@ -82,8 +82,8 @@
                         {% if dom_caps.cpu_modes %}
                         <div class="col-md-6">
                             <label for="vm_vcpu_mode" class="form-label">{% trans "CPU mode" %}</label>
-                            <select id="vm_vcpu_mode" class="form-select{{ vm_form.vcpu_mode|invalid }}" name="vcpu_mode"{% error_attrs vm_form.vcpu_mode "vm_vcpu_mode" %}>
-                                <option value="">{% trans 'no-mode' %}</option>
+                            <select id="vm_vcpu_mode" class="form-select{{ vm_form.vcpu_mode|invalid }}" name="vcpu_mode" required{% error_attrs vm_form.vcpu_mode "vm_vcpu_mode" %}>
+                                {% if not vm_form.vcpu_mode.value %}<option value="" selected disabled>{% trans "Choose…" %}</option>{% endif %}
                                 {% for mode in dom_caps.cpu_modes %}
                                     {% if mode == 'custom' %}
                                         <optgroup label="{% trans "Custom CPU Models" %}">