(2.10) Return link_name in blockdev_assemble rpc

(2.10) Return link_name in blockdev_assemble rpc

Until now this RPC returned only dev_path. Since we use it in
hotplug we have to know the simlink of the device so that we
pass it to the corresponding hypervisor command and include it
in block_devices entry in runtime files.

Fix unittest to be aware of changed rpc

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>

cmdlib unittest do not exist in stable-2.8

Conflicts:
	test/py/cmdlib/instance_migration_unittest.py
	test/py/cmdlib/instance_unittest.py

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>