kvm: Passthrough host CPU in order to allow nesting

Before this patch the virtual CPU was the stock qemu CPU, in the sense that
the virtual CPU features were set according to the default qemu CPU.
With this change the CPU features of the host will be copied at start to the
domain definition. This includes features like svm and vmx, which in turn allow
to run nested virtualization if the host is configured accordingly i.e.
`kvm_intel nested=y` in `/etc/modprobe.d/kvm.conf`.

Resolves #kubernetes#2553

Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>

pkg/drivers/kvm/domain.go

@@ -37,6 +37,7 @@ const domainTmpl = `
     <apic/>
     <pae/>
   </features>
+  <cpu mode='host-passthrough'></cpu>
   <os>
     <type>hvm</type>
     <boot dev='cdrom'/>