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>