Refactor updateVmPolicy to use DOM manipulation

This allows updating the old values instead of just setting
new data. It will be needed when more sections than cputune
are present in the metadata xml, because libvirt can only set
the full metadata block (no partial updates).

Change-Id: I475a70269c1f32e10f379b0743cfeefe642d4afd
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28895
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>