nodedev: Introduce the mdev capability to a PCI parent device

nodedev: Introduce the mdev capability to a PCI parent device

The parent device needs to report the generic stuff about the supported
mediated devices types, like device API, available instances, type name,
etc. Therefore this patch introduces a new nested capability element of
type 'mdev_types' with the resulting XML of the following format:

<device>
  ...
  <capability type='pci'>
    ...
    <capability type='mdev_types'>
      <type id='vendor_supplied_id'>
        <name>optional_vendor_supplied_codename</name>
        <deviceAPI>vfio-pci</deviceAPI>
        <availableInstances>NUM</availableInstances>
      </type>
        ...
      <type>
        ...
      </type>
    </capability>
  </capability>
  ...
</device>

https://bugzilla.redhat.com/show_bug.cgi?id=1452072

Signed-off-by: Erik Skultety <eskultet@redhat.com>