net/dump: Issue a warning for the deprecated "-net dump"

Network dumping should be done with "-object filter-dump" nowadays.
Using "-net dump" via the VLAN mechanism is considered as deprecated
and might be removed in a future release. So warn the users now
to inform them to user the filter-dump method instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

net/dump.c

@@ -194,6 +194,9 @@ int net_init_dump(const Netdev *netdev, const char *name,
 
     assert(peer);
 
+    error_report("'-net dump' is deprecated. "
+                 "Please use '-object filter-dump' instead.");
+
     if (dump->has_file) {
         file = dump->file;
     } else {