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_instance.py

@@ -93,7 +93,7 @@ class CreateInstanceViewTests(TestCase):
         conn.get_nwfilters.return_value = []
         conn.get_network_models.return_value = ["virtio"]
         conn.get_storages.return_value = ["default"]
-        conn.get_dom_capabilities.return_value = {"loader_enums": [], "loaders": []}
+        conn.get_dom_capabilities.return_value = {"loader_enums": [], "loaders": [], "cpu_modes": ["host-model"]}
         conn.get_capabilities.return_value = {}
         conn.label_for_firmware_path.return_value = None
         conn.find_uefi_path_for_arch.return_value = None
@@ -136,7 +136,7 @@ class CreateInstanceViewTests(TestCase):
         conn.get_nwfilters.return_value = []
         conn.get_cache_modes.return_value = {}
         # A host without loader support has no "loaders"/"loader_enums" keys
-        conn.get_dom_capabilities.return_value = {"loader_support": "no"}
+        conn.get_dom_capabilities.return_value = {"loader_support": "no", "cpu_modes": ["host-model"]}
         conn.get_capabilities.return_value = {}
         conn.label_for_firmware_path.return_value = None
         conn.find_uefi_path_for_arch.return_value = None