appliance: Use <cpu mode="maximum"/> for -cpu max on libvirt.

Note this requires libvirt >= 7.1.0 which was only released in March 2021.

With an older libvirt you will see this error:

  Original error from libvirt: unsupported configuration: Invalid mode attribute 'maximum' [code=67 int1=-1]

In theory we could check if this is supported by looking at the
libvirt capabilities and fall back, but this commit does not do that,
in the expectation that most people will be using the default backend
(direct) and on Fedora/RHEL we will add an explicit minimum version
dependency to the package.

qemu support has been around quite a bit longer (at least since 2017).

Fixes: commit 30f74f38bd

lib/launch-libvirt.c

@@ -1183,13 +1183,8 @@ construct_libvirt_xml_cpu (guestfs_h *g,
         } end_element ();
       }
       else if (STREQ (cpu_model, "max")) {
-        if (params->data->is_kvm)
-          attribute ("mode", "host-passthrough");
-        else
-          attribute ("mode", "host-model");
-        start_element ("model") {
-          attribute ("fallback", "allow");
-        } end_element ();
+        /* https://bugzilla.redhat.com/show_bug.cgi?id=1935572#c11 */
+        attribute ("mode", "maximum");
       }
       else
         single_element ("model", cpu_model);