This enrichs HBA's xml by dumping the number of max vports and
vports in use. Format is like:

  <capability type='vport_ops'>
    <max_vports>164</max_vports>
    <vports>5</vports>
  </capability>

* docs/formatnode.html.in: (Document the new XML)
* docs/schemas/nodedev.rng: (Add the schema)
* src/conf/node_device_conf.h: (New member for data.scsi_host)
* src/node_device/node_device_linux_sysfs.c: (Collect the value of
  max_vports and vports)