Adding network api xml support

Network API tests with xml formatted request body were missing.
I added code to support this feature.
In the test cases "test_list_subnets" and "test_list_networks",
response code check was also missing.
I also added this check in the code.

Change-Id: I876c720b93e0d0cded96de0549edd6115fa1c655
Merge "Adding network api xml support"