Update live migration test to use new syntax for create_server

Needed due to change I208b266253c0e48da66d1dd4c84a3c074c38d1a6
which modified the return value and required a new argument

Change-Id: Ic76e375af1ac6bbf2486677706917c8064ff1658
Fixes: bug 1135465
diff --git a/tempest/tests/compute/test_live_block_migration.py b/tempest/tests/compute/test_live_block_migration.py
index dcd6a78..f2ec753 100644
--- a/tempest/tests/compute/test_live_block_migration.py
+++ b/tempest/tests/compute/test_live_block_migration.py
@@ -86,7 +86,7 @@
             if 'ACTIVE' == self._get_server_status(server_id):
                 return server_id
         else:
-            server = self.create_server()
+            _, server = self.create_server(wait_until="ACTIVE")
             server_id = server['id']
             self.password = server['adminPass']
             self.password = 'password'