Latest kernel check fails if repoquery fails

Currently, the is_loaded_kernel_latest() check tracebacks with "ValueError: need more than 1 value to unpack" if the repoquery command called in the check fails for any reason. For example when the system has defined a repository that is not accessible ("[Errno 14] HTTP Error 404 - Not Found").

The problem is that we don't check the return code of the repoquery call, we move on expecting that the repoquery returned a list of available kernels, and then it tracebacks on accessing the list.

Link to a log file of the failing test: http://artifacts.osci.redhat.com/testing-farm/ab58e443-1f4c-4804-bb42-390706db6235/work-changed_grub_validzjVTid/log.txt (backup: http://pastebin.test.redhat.com/1072699).