nodedev: add mdev support to virNodeDeviceCreateXML()

nodedev: add mdev support to virNodeDeviceCreateXML()

With recent additions to the node device xml schema, an xml schema can
now describe a mdev device sufficiently for libvirt to create and start
the device using the mdevctl utility.

Note that some of the the configuration for a mediated device must be
passed to mdevctl as a JSON-formatted file. In order to avoid creating
and cleaning up temporary files, the JSON is instead fed to stdin and we
pass the filename /dev/stdin to mdevctl. While this may not be portable,
neither are mediated devices, so I don't believe it should cause any
problems.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>