Add best practices properties to MS Win image example

Add properties to the Windows image example which are best practices:
- Add os-distro
- Add os-version
- Revise os-variant to match other Windows values 8k > 12k

Change-Id: Id829df0a83ac6d32191eb2f13f9873950f7f71a6
Closes-bug: #1663330

doc/image-guide/source/windows-image.rst

@@ -31,7 +31,8 @@ using the :command:`virt-install` command and the KVM hypervisor.
         --disk path=ws2012.qcow2,format=qcow2,device=disk,bus=virtio \
         --cdrom /path/to/en_windows_server_2012_x64_dvd.iso \
         --disk path=/path/to/virtio-win-0.1-XX.iso,device=cdrom \
-        --vnc --os-type windows --os-variant win2k8
+        --vnc --os-type windows --os-variant win2k12 \
+        --os-distro windows --os-version 2012
 
    Use :command:`virt-manager` or :command:`virt-viewer` to
    connect to the VM and start the Windows installation.