qemu: stop passing in virConnectPtr for looking up networks

When setting up graphics, we sometimes need to resolve networks,
requiring the caller to pass in a virConnectPtr, except sometimes they
pass in NULL. Use virGetConnectNetwork() to acquire the connection to
the network driver when it is needed.

Reviewed-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

tests/qemuxml2argvtest.c

@@ -498,7 +498,7 @@ testCompareXMLToArgv(const void *data)
         }
     }
 
-    if (!(cmd = qemuProcessCreatePretendCmd(conn, &driver, vm, migrateURI,
+    if (!(cmd = qemuProcessCreatePretendCmd(&driver, vm, migrateURI,
                                             (flags & FLAG_FIPS), false,
                                             VIR_QEMU_PROCESS_START_COLD))) {
         if (flags & FLAG_EXPECT_FAILURE)