network: Add virNetworkObj Get/Set API's for @def and @newDef

network: Add virNetworkObj Get/Set API's for @def and @newDef

In preparation for making the object private, create a couple of API's
to get the obj->def & obj->newDef and set the obj->def.

While altering networkxml2conftest.c to use the virNetworkObjSetDef
API, fix the name of the variable from @dev to @def

Signed-off-by: John Ferlan <jferlan@redhat.com>