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/tests/test_create_rollback.py

@@ -33,6 +33,7 @@ class CreateRollbackTestCase(TestCase):
         with patch("instances.jobs.wvmCreate") as wvm:
             conn = wvm.return_value
             conn.get_cache_modes.return_value = {"none": "Disable cache"}
+            conn.get_dom_capabilities.return_value = {"cpu_modes": ["host-model"]}
             conn.get_instances.return_value = []
             conn.create_volume.return_value = "/pool/rb-vm.qcow2"
             op = jobs.submit("instance.create", self.admin, {**PARAMS, "compute": self.compute.pk}, "key-rollback1")