Use correct image variable for vm install

Before we used the default image variable. Overriding the image did not
properly work. Now it does!

tasks/install-vm.yml

@@ -77,7 +77,7 @@
 
 # qemu-img for image
 - name: Write image
-  shell: "qemu-img convert -O raw {{ xen_vm_creator_image|quote }} /dev/{{ xen_vm_creator_lvm_vg_name }}/{{ vm.hostname }}-disk"
+  shell: "qemu-img convert -O raw {{ vm.image|quote }} /dev/{{ xen_vm_creator_lvm_vg_name }}/{{ vm.hostname }}-disk"
 
 # start vm
 - name: Start VM