We should not raise any interrupt after VM has been stopped but this
is what e1000 currently did when mit timer is active in
pre_save(). Fixing this by scheduling a timer in post_load() which can
make sure the interrupt was raised when VM is running.

Reported-and-tested-by: Longpeng <longpeng2@huawei.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>