hw/virtio-net.c: set config size using host features

Currently, the config size for virtio devices is hard coded. When a new
feature is added that changes the config size, drivers that assume a static
config size will break. For purposes of backward compatibility, there needs
to be a way to inform drivers of the config size needed to accommodate the
set of features enabled.

aliguori: merged in
 - hw/virtio-net: use existing macros to implement endof
 - hw/virtio-net: fix config_size data type

Signed-off-by: Jesse Larrew <jlarrew@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>