nodedev: Introduce mdev capability for mediated devices

nodedev: Introduce mdev capability for mediated devices

Start discovering the mediated devices on the host system and format the
attributes for the mediated device into the XML. Compared to the parent
device which reports generic information about the abstract mediated
devices types, a child device only reports the type name it has been
instantiated from and the IOMMU group number, since that's device
specific compared to the rest of the info that can be gathered about
mediated devices at the moment.
This patch introduces both the formatting and parsing routines, updates
nodedev.rng schema, adding a testcase as well.

The resulting mdev child device XML:
<device>
  <name>mdev_4b20d080_1b54_4048_85b3_a6a62d165c01</name>
  <path>/sys/devices/.../4b20d080-1b54-4048-85b3-a6a62d165c01</path>
  <parent>pci_0000_06_00_0</parent>
  <driver>
    <name>vfio_mdev</name>
  </driver>
  <capability type='mdev'>
    <type id='vendor_supplied_type_id'/>
    <iommuGroup number='NUM'/>
  <capability/>
<device/>

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

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