KVM: configure bridged NICs at migration start

Commit 5d9bfd87 moved tap interface handling from KVM to Ganeti, partly to
also solve the problem of routed interfaces getting configured too early during
live migrations, causing network anomalies. In that direction, configuration of
NICs of incoming instances was deferred to FinalizeMigration time.

However, this causes minor issues with bridged interfaces; KVM sends out an
ARP-like packet upon migration finish, which is lost because the tap interface
is not yet configured. As a consequence, intermediate network equipment (i.e.
switches) does not get notified about the topology change, until the instance
transmits another packet after the bridge has been configured, or the switch's
ARP cache expires.

The proper solution to that is to support different phases in network
configuration (pre/post migration), which also requires separate ifup scripts.
Until then we fall back to configuring bridged interfaces on incoming instances
at migration start, instead of finish.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>