virInterface: Expose link state & speed

virInterface: Expose link state & speed

Currently it is not possible to determine the speed of an interface
and whether a link is actually detected from the API. Orchestrating
platforms want to be able to determine when the link has failed and
where multiple speeds may be available which one the interface is
actually connected at. This commit introduces an extension to our
interface XML (without implementation to interface driver backends):

  <interface type='ethernet' name='eth0'>
    <start mode='none'/>
    <mac address='aa:bb:cc:dd:ee:ff'/>
    <link speed='1000' state='up'/>
    <mtu size='1492'/>
    ...
  </interface>

Where @speed is negotiated link speed in Mbits per second, and state
is the current NIC state (can be one of the following:  "unknown",
"notpresent", "down", "lowerlayerdown","testing", "dormant", "up").

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>