virt: move __hacked_writexml in the vmxml module

No reason to keep this hack under the spotlight,
and give to vm.py some much needed relief.
No code changes.

Change-Id: I08b45cf857a79b4dd2624112f690e3f1221422d5
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29694
Reviewed-by: Dan Kenigsberg <danken@redhat.com>