feat(instances): a VM's options in one list, each saved when it can be

Settings > Options was four forms under bold sentences, most of them
disabled while the VM ran. The owner chose this layout from three
rendered options: one list, each row a setting with its description on
the left and its control on the right: VM details (title, description),
Template, primary video model, QEMU guest agent. vCPU hot plug moved to
Resize > CPU, next to the vCPU counts.

Codex reviewed the four forms; these are fixed:

- Title, description and the video model are saved while the VM runs:
  they are in the persistent definition, used at the next start.
- The template flag has its own form and view, set_template, for
  superusers and staff, and only for a shut-off VM: a running VM made a
  template left the quota while it kept running. Turning a template
  back into a VM checks the quota there, and each change is audited.
  The title form no longer touches the flag.
- A new video model stays the primary one; the old model's memory and
  head settings, which are its own, go with it.
- The agent and vCPU hot plug forms show the persistent definition; they
  showed the running VM's, so a pending change looked undone.
- Enabling the guest agent adds its channel only where it is missing:
  saving Enabled again failed, and a channel pending the next start kept
  a running VM from getting one.
- The agent line tells a VM that is not running, one without a channel,
  and a connected or not connected agent apart.

instances/models.py

@@ -265,6 +265,16 @@ class Instance(models.Model):
     def guest_agent(self):
         return False if self.proxy.get_guest_agent() is None else True
 
+    @cached_property
+    def config_guest_agent(self) -> bool:
+        """Whether the next start has the agent channel; the agent form shows it."""
+        return self.proxy.get_guest_agent(config=True) is not None
+
+    @cached_property
+    def config_vcpu_hotplug(self) -> bool:
+        """Whether the next start has hotpluggable vCPUs; the hot plug form shows it."""
+        return any(vcpu["hotpluggable"] == "yes" for vcpu in self.proxy.get_vcpus(config=True).values())
+
     @cached_property
     def guest_agent_ready(self) -> bool:
         return self.proxy.is_agent_ready()