XenAPI: Return platform_version if no product_version

Nova now requires a version to be set.  In the case of xapi-XCP/xenserver-core
no product_version exists, so use the platform_version instead.

Fixes bug 1206948

Change-Id: I4d6008216075716c8b969493b7a2ccb7713f755d
Merge "XenAPI: Return platform_version if no product_version"