networkxml2xmltest: Dynamically allocate testInfo struct

networkxml2xmltest: Dynamically allocate testInfo struct

So far, the testInfo struct contained immutable data (from its
lifetime point of view). But that is about to change. For
instance, it will hold parsed network definition (virNetworkDef)
and in order to avoid leaking dynamically allocated data
corresponding free function must be introduced (or clear
function, doesn't really matter). At this point, the structure
might as well be dynamically allocated entirely.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>