virCaps: Expose distance between host NUMA nodes

If user or management application wants to create a guest,
it may be useful to know the cost of internode latencies
before the guest resources are pinned. For example:

<capabilities>

  <host>
    ...
    <topology>
      <cells num='2'>
        <cell id='0'>
          <memory unit='KiB'>4004132</memory>
          <distances>
            <sibling id='0' value='10'/>
            <sibling id='1' value='20'/>
          </distances>
          <cpus num='2'>
            <cpu id='0' socket_id='0' core_id='0' siblings='0'/>
            <cpu id='2' socket_id='0' core_id='2' siblings='2'/>
          </cpus>
        </cell>
        <cell id='1'>
          <memory unit='KiB'>4030064</memory>
          <distances>
            <sibling id='0' value='20'/>
            <sibling id='1' value='10'/>
          </distances>
          <cpus num='2'>
            <cpu id='1' socket_id='0' core_id='0' siblings='1'/>
            <cpu id='3' socket_id='0' core_id='2' siblings='3'/>
          </cpus>
        </cell>
      </cells>
    </topology>
    ...
  </host>
  ...
</capabilities>

We can see the distance from node1 to node0 is 20 and within nodes 10.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

docs/schemas/capability.rng

@@ -188,6 +188,21 @@
         <ref name='memory'/>
       </optional>
 
+      <optional>
+        <element name='distances'>
+          <zeroOrMore>
+            <element name='sibling'>
+              <attribute name='id'>
+                <ref name='unsignedInt'/>
+              </attribute>
+              <attribute name='value'>
+                <ref name='unsignedInt'/>
+              </attribute>
+            </element>
+          </zeroOrMore>
+        </element>
+      </optional>
+
       <optional>
         <element name='cpus'>
           <attribute name='num'>