conf: make virDomainNetDefFormat a public function

We will need to call virDomainNetDefFormat() from the network hook (in
the network driver).

src/conf/domain_conf.c

@@ -15504,7 +15504,7 @@ virDomainActualNetDefFormat(virBufferPtr buf,
     return 0;
 }
 
-static int
+int
 virDomainNetDefFormat(virBufferPtr buf,
                       virDomainNetDefPtr def,
                       unsigned int flags)