lib: Enable ACPI for the libvirt backend

Many years ago we used to pass acpi=off on the Linux kernel command
line.  In commit db1f811b2 we stopped doing that (around 2016).
However unless you also use:

  <features>
    <acpi/>
  </features>

then it turns out that libvirt disables ACPI generation at the qemu
level.  None of this mattered until SeaBIOS 1.17 changed its
behaviour, causing ACPI to be required for virtio devices to work.

Updates: commit db1f811b29
Related: https://bugzilla.redhat.com/show_bug.cgi?id=2372329
Thanks: Gerd Hoffmann

lib/launch-libvirt.c

@@ -1175,6 +1175,10 @@ construct_libvirt_xml_cpu (guestfs_h *g,
 
   single_element_format ("vcpu", "%d", g->smp);
 
+  start_element ("features") {
+    empty_element ("acpi");
+  } end_element ();
+
   start_element ("clock") {
     attribute ("offset", "utc");