libxl: report numa sibling distances on host capabilities

libxl: report numa sibling distances on host capabilities

When running on a NUMA machine, populate the sibling node
and distance information using data supplied by Xen.

With locality distances information, under Xen, new host
capabilities would like:

    <topology>
      <cells num='4'>
        <cell id='0'>
          <memory unit='KiB'>263902380</memory>
          <distances>
            <sibling id='0' value='10'/>
            <sibling id='1' value='21'/>
          </distances>
          ...
        </cell>
        ...
      </cells>
      ...
    </topology>

Signed-off-by: Wim ten Have <wim.ten.have@oracle.com>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>