feat(instances): a VM's migration in a route, a disk plan and its transfer settings

Settings > Migrate was a horizontal form of six switches without
explanations. The owner chose this layout from three rendered options:
the route on top as "From" (the source host) and "To" (the
destination select) side by side, a "Storage" card with
where each disk goes, a "Transfer settings" card with the switches in
two columns, each with a line saying what it does, and a footer.

Codex reviewed migration end to end; these are fixed:

- No Offline switch: the mode follows the VM's state, and one line says
  which. A shut-off VM moved offline whether the switch was on or off.
- Live migration shows for a running or paused VM; Auto converge,
  Compressed and Unsafe only for a running VM, the only state the
  migration applies them to. Unsafe was offered for a paused or shut-off
  VM and ignored.
- Unsafe says it skips libvirt's safety checks; Delete source files
  says it deletes only the files of copied disks.
- A VM that cannot migrate anywhere (no other host, a state that does
  not migrate, snapshots, host devices) gets the reason and a disabled
  button instead of the disk plan and the switches; it learned that
  after submitting.
- Migrate stays disabled while the destination did not answer or a disk
  cannot go there; it was enabled after any plan.

instances/templates/instance.html

@@ -409,7 +409,8 @@
             return response.text();
         }).then(function (html) {
             migratePlan.innerHTML = html;
-            submit.disabled = false;
+            // a destination that did not answer, or a disk that cannot go there
+            submit.disabled = migratePlan.querySelector("[data-refused]") !== null;
         }).catch(function () {
             if (loading.signal.aborted) return;  // another destination was chosen
             migratePlan.textContent = "{{ _("The disks could not be checked.")|escapejs }}";