Commit "e1000e: Prevent MSI/MSI-X storms" introduced msi_causes_pending
to prevent interrupt storms problem. It was tested with MSI-X.

In case of MSI, the guest can rely solely on interrupts to clear ICR.
Upon clearing all pending interrupts, msi_causes_pending gets cleared.
However, when e1000e_itr_should_postpone() in e1000e_send_msi() returns
true, MSI never gets fired by e1000e_intrmgr_on_throttling_timer()
because msi_causes_pending is still set. This results in interrupt loss.

To prevent this, we need to clear msi_causes_pending when MSI is going
to get fired by the throttling timer. The guest can then receive
interrupts eventually.

Signed-off-by: Ake Koomsin <ake@igel.co.jp>
Signed-off-by: Jason Wang <jasowang@redhat.com>