prefs: Allow changing the default VM CPU mode/model config

Add a preference for changing the default CPU mode/model. The options are:

- default (whatever virt-manager chooses as the default)
- hypervisor default (no <cpu> block, what we've always done)
- nearest host cpu model (just the 'model' from caps->host->cpu)
- copy host cpu (the entire <cpu> block from caps->host)

The setting only applies to KVM guests, not tested with anything else.
The default is left as is for now (hypervisor default).

copy host CPU will one day use mode='host-model', when it does what
we want. At that point we will probably make it the default

data/org.virt-manager.virt-manager.gschema.xml

@@ -207,6 +207,13 @@
       <summary>Use selected format for new VM storage</summary>
       <description>Use selected format when creating new disk images in new VM wizard</description>
     </key>
+
+    <key name="cpu-default" type="s">
+      <default>'default'</default>
+      <summary>CPU setting to use for new VMs</summary>
+      <description>CPU setting to use for new VMs. Limited to VMs matching the host architecture. Possible values: default (virt-manager default), hv-default (qemu's default), host-cpu-model (just the model, not the additional features), host-model (libvirt's host-model setting).</description>
+    </key>
+
   </schema>
 
   <schema id="org.virt-manager.virt-manager.paths"