qemu.control.kernel-version: Bug fix for control.kernel-version

This patch fix three problems in control.kernel-version:
  - Fix the wrong variable name.
  - Remove the parse_file inside control file for host-kernel.cfg
    Just in clude it as other cfg files in your tests-example.cfg
    which is more flexible. And this also can work with the situation
    that without host-kernel.cfg.
  - Don't get the package version if there is no need for package
    version check.

Also add some docstring for how to use the package filter.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>