qemu: Add support for host CPU modes

This adds support for host-model and host-passthrough CPU modes to qemu
driver. The host-passthrough mode is mapped to -cpu host.

tests/qemuxml2argvdata/qemuxml2argv-cpu-host-model-nofallback.xml

@@ -0,0 +1,21 @@
+<domain type='qemu'>
+  <name>QEMUGuest1</name>
+  <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
+  <memory>219100</memory>
+  <currentMemory>219100</currentMemory>
+  <vcpu>6</vcpu>
+  <os>
+    <type arch='x86_64' machine='pc'>hvm</type>
+    <boot dev='network'/>
+  </os>
+  <cpu mode='host-model'>
+    <model fallback='forbid'/>
+  </cpu>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/./qemu-supported-cpus.sh</emulator>
+  </devices>
+</domain>