'virsh capabilities' will now include a new <memory> element
per <cell> of the topology, as in:

    <topology>
      <cells num='2'>
        <cell id='0'>
          <memory unit='KiB'>12572412</memory>
          <cpus num='12'>
          ...
        </cell>
Signed-off-by: NEric Blake <eblake@redhat.com>