src: network_conf: propagate only bool to virNetworkDefParseString()

src: network_conf: propagate only bool to virNetworkDefParseString()

We don't need to propagate all public flags, only the information
about the presence of the validation one, which can differ from
function to function. This patch makes it easier and more
readable in case of a future additions of validation flags.
This change was suggested by Daniel.

Signed-off-by: Kristina Hanicova <khanicov@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>