Fix bridging in net-common

Fix bridging in net-common

Bridging does not work correctly in net-common. The parameters passed
by Ganeti are put in /var/run/ganeti/xen-hypervisor/nic/$domname/$devid
and read again from there before running the setup functions according
to the $MODE which is read from the file.

When using MODE=bridged, the script expects to get the name of the
bridge passed as $BRIDGE, but is actually stored and passed as $LINK.
This patch fixes the issue.

Signed-off-by: Sebastian Gebhard <sege@fs.ei.tum.de>
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>