virt.virtnet : update vm.virtnet when guest reboot

When start a guest with one nic device, then start a guest with two
nics(don't remove env), the second nic can not be init correctly.
When boot guest with nic_extra_params, then boot another guest w/o
this option, the second guest will boot with nic_extra_params
unexpectedly.
The root cause of these issues is we don't update virtnet when
preprocess VM object

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>