[Property] Code cleaning

Remove useless pointer check.

Update prv_add_strv_prop to behave as other functions.
Now it checks if 'value' parameter is not NULL.

Signed-off-by: Ludovic Ferrandis <ludovic.ferrandis@intel.com>
[Property] Display Resource property in Container

Fix https://github.com/01org/dleyna-server/issues/5

Add 'Resource' property
Add GetCompatibleResource API

Signed-off-by: Ludovic Ferrandis <ludovic.ferrandis@intel.com>
[Bug Fix] Fix a memory leak in dls_upnp_get_resource

Signed-off-by: Ludovic Ferrandis <ludovic.ferrandis@intel.com>