CLOUDSTACK-1638: Introduce NetworkMigrationResponder

The location of the virtual machine is provided by DeployDestination, which will
be passed in NetworkGuru#reserve and NetworkElement#prepare.

During the virtual machine migration, it actually changes DeployDestination and
it looks like that it will tell that event to network components as it has
NetworkManager#prepareNicForMigration. The problem is that althogh the interface
has that method, NetworkManagerImpl does not tell the DeployDestination changes
to network components.

So IMHO, we need to add calls of NetworkGuru#reserve and NetworkElement#prepare
in NetworkManagerImpl#prepareNicForMigration . And then, we also need to add
calls NetworkGuru#release and NetworkElement#release after the migration,
otherwise the network resources that plugin reserved will be kept even when the
vm leaves off.

(Sheng Yang: rebase code, add license header)

Signed-off-by: Sheng Yang <sheng.yang@citrix.com>
PVLAN: CLOUDSTACK-2401: VM Migration support