vmdesc: Add minimal support for CPU model selection

The `cpu` property of `VirtualMachine` objects indicates the value of
the `mode` attribute of the `cpu` element in the domain XML
configuration. The possible values are

* host-model (default)
* host-passthrough
* custom

src/mkvm/vmdesc.py

@@ -74,6 +74,7 @@ class VirtualMachine(object):
         'fqdn',
         'ram',
         'vcpus',
+        'cpu',
         'net_ifaces',
         'console',
         'graphics',
@@ -101,6 +102,7 @@ class VirtualMachine(object):
         self.fqdn = None
         self.ram = 256 * 2 ** 20
         self.vcpus = 1
+        self.cpu = 'host-model'
         self.net_ifaces = []
         self.console = 'virtio'
         self.graphics = 'spice'
@@ -279,6 +281,9 @@ class VirtualMachine(object):
         elm_vcpu = etree.SubElement(root, 'vcpu', placement='static')
         elm_vcpu.text = str(self.vcpus)
 
+        if self.cpu:
+            elm_cpu = etree.SubElement(root, 'cpu', mode=self.cpu)
+
         elm_os = etree.SubElement(root, 'os')
         elm_type = etree.SubElement(elm_os, 'type', arch='x86_64')
         elm_type.text = 'hvm'