Though numad will manage the memory allocation of task dynamically,
it wants management application (libvirt) to pre-set the memory
policy according to the advisory nodeset returned from querying numad,
(just like pre-bind CPU nodeset for domain process), and thus the
performance could benefit much more from it.

This patch introduces new XML tag 'placement', value 'auto' indicates
whether to set the memory policy with the advisory nodeset from numad,
and its value defaults to the value of <vcpu> placement, or 'static'
if 'nodeset' is specified. Example of the new XML tag's usage:

  <numatune>
    <memory placement='auto' mode='interleave'/>
  </numatune>

Just like what current "numatune" does, the 'auto' numa memory policy
setting uses libnuma's API too.

If <vcpu> "placement" is "auto", and <numatune> is not specified
explicitly, a default <numatume> will be added with "placement"
set as "auto", and "mode" set as "strict".

The following XML can now fully drive numad:

1) <vcpu> placement is 'auto', no <numatune> is specified.

   <vcpu placement='auto'>10</vcpu>

2) <vcpu> placement is 'auto', no 'placement' is specified for
   <numatune>.

   <vcpu placement='auto'>10</vcpu>
   <numatune>
     <memory mode='interleave'/>
   </numatune>

And it's also able to control the CPU placement and memory policy
independently. e.g.

1) <vcpu> placement is 'auto', and <numatune> placement is 'static'

   <vcpu placement='auto'>10</vcpu>
   <numatune>
     <memory mode='strict' nodeset='0-10,^7'/>
   </numatune>

2) <vcpu> placement is 'static', and <numatune> placement is 'auto'

   <vcpu placement='static' cpuset='0-24,^12'>10</vcpu>
   <numatune>
     <memory mode='interleave' placement='auto'/>
   </numatume>

A follow up patch will change the XML formatting codes to always output
'placement' for <vcpu>, even it's 'static'.