feat(instances): a VM's boot settings as switches and its boot order as one list

The Boot pane shows Autostart and the boot menu as switches that save on
change, and the boot order as one list of the VM's disks, CD-ROM drives
and NICs: the devices in the order first, checked and numbered, the
others below. Up and Down move the checked devices; Apply order saves.

- The boot menu can be changed while the VM runs: it is in the
  persistent definition, so it applies at the next start, as the boot
  order does.
- The boot order editor no longer needs the boot menu enabled.
- A VM that boots by device type (<os><boot dev='hd'/>) names those
  types; checking devices replaces that order.
- An order is refused while a device the list does not show (a host
  device, a floppy) has a boot place, so Apply does not drop it; the
  message points to the XML editor.
- An empty order is refused with a message instead of being ignored.
- get_bootorder gives a NIC's full MAC as its target.
- The unused jQuery multiselect plugin is removed.

static/css/webvirtcloud.css

@@ -102,3 +102,25 @@ th[aria-sort="ascending"]:not([data-sort-method="none"])::after {
 .host-nav .breadcrumb-item + .breadcrumb-item::before {
     padding-right: var(--bs-breadcrumb-item-padding-x);
 }
+
+/* the boot order editor numbers its checked devices, top to bottom */
+.boot-sequence {
+    counter-reset: boot;
+}
+
+.boot-sequence > label:has(> input:checked) {
+    counter-increment: boot;
+}
+
+.boot-rank {
+    width: 1rem;
+    text-align: center;
+}
+
+.boot-rank::before {
+    content: "—";
+}
+
+.boot-sequence > label:has(> input:checked) .boot-rank::before {
+    content: counter(boot);
+}