hw/net: Make NetCanReceive() return a boolean

hw/net: Make NetCanReceive() return a boolean

The NetCanReceive handler return whether the device can or
can not receive new packets. Make it obvious by returning
a boolean type.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>