Support for "hostdev" / VFIO devices

[lklimek]

Hi,

great work with this exporter!

Unfortunately, I've hit a bug. When the exporter finds in domain XML definition of a device without target - for example, VFIO interfaces which can be detected as "hostdev" devices - it fails with error:

libvirt_exporter[20034]: 2017/11/24 10:22:11 Failed to scrape metrics: virError(Code=8, Domain=10, Message='invalid argument: invalid path, '' is not a known interface')

and the exporter stops the processing.

Expected behavior - warn about lack of device and ignore the interface (or gather stats in some different way if possible).

I believe this is the fragment of our domain xml file that causes this issue:

<interface type='hostdev' managed='yes'>
  <mac address='AA:BB:CC:DD:EE:FF'/>
  <driver name='vfio'/>
  <source>
    <address type='pci' domain='0x0000' bus='0x04' slot='0x10' function='0x4'/>
  </source>
  <model type='virtio'/>
  <alias name='hostdev0'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
</interface>