tap: forbid creating multiqueue tap when hub is used

Obviously, hub does not support multiqueue tap. So this patch forbids creating
multiple queue tap when hub is used to prevent the crash when command line such
as "-net tap,queues=2" is used.

Cc: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
