Fixed integer overflow in e1000e

Fixed integer overflow in e1000e

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1737400
Fixed setting max_queue_num if there are no peers in
NICConf. qemu_new_nic() creates NICState with 1 NetClientState(index
0) without peers, set max_queue_num to 0 - It prevents undefined
behavior and possible crashes, especially during pcie hotplug.

Fixes: 6f3fbe4ed06 ("net: Introduce e1000e device emulation")
Signed-off-by: Andrew Melnychenko <andrew@daynix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Dmitry Fleytman <dmitry.fleytman@gmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>