interface: report link state for bonds and vlans too

The interface state for bonds and vlans does seem to reflect the state
of the underlying physical devices, at least in some cases, so it
makes sense to allow reporting it (netcf now does).

The link state/speed for bridge devices is meaningless though, so we
don't even look for it.

tests/interfaceschemadata/bond.xml

@@ -1,5 +1,6 @@
 <interface type='bond' name='bond0'>
   <start mode='none'/>
+  <link speed='1000' state='up'/>
   <protocol family='ipv4'>
     <ip address='192.168.50.7' prefix='24'/>
     <route gateway='192.168.50.1'/>

tests/interfaceschemadata/vlan.xml

@@ -1,5 +1,6 @@
 <interface type='vlan' name='eth0.42'>
   <start mode='onboot'/>
+  <link state='lowerlayerdown'/>
   <protocol family='ipv4'>
     <dhcp peerdns='no'/>
   </protocol>