vm: Create vm numa node according to the node index

For vm numa node in libvirt, the node index is decided by the cell
sequence of the configuration xml. So we sort the node items according
to the node index and build the dom xml with the sequence.

Change-Id: I622e561a72fced21f0655c5de0ea520adfb0612a
Bug-Url: https://bugzilla.redhat.com/1099814
Signed-off-by: Bruce Shi <xiao-lei.shi@hp.com>
Reviewed-on: http://gerrit.ovirt.org/27987
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>