create_libvirt_xml: honor "gcaps_default_cpu"

If "s_cpu_model" is None and "gcaps_default_cpu" is "false", generate the
<cpu mode="host-passthrough"/> element for libvirt.

This element produces an (almost) exact copy of the host (i.e., physical)
CPU for the guest, which is the best choice for guest OSes that cannot run
on QEMU's default VCPU type -- considering that domains converted by
virt-v2v are not expected to be migrateable without further tweaks by an
administrator.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2076013
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20220420162333.24069-7-lersek@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>

output/create_libvirt_xml.ml

@@ -181,12 +181,15 @@ let create_libvirt_xml ?pool source inspect
   ];
 
   if source.s_cpu_model <> None ||
+     not guestcaps.gcaps_default_cpu ||
      source.s_cpu_topology <> None then (
     let cpu_attrs = ref []
     and cpu = ref [] in
 
     (match source.s_cpu_model with
-     | None -> ()
+     | None ->
+         if not guestcaps.gcaps_default_cpu then
+           List.push_back cpu_attrs ("mode", "host-passthrough");
      | Some model ->
          List.push_back cpu_attrs ("match", "minimum");
          (match source.s_cpu_vendor with