e1000: fix hang of win2k12 shutdown with flood ping

e1000 driver in Win2k12 is really well rotten. It 100% hangs on shutdown
of UP VM under flood ping. The guest checks card state and reinjects
itself interrupt in a loop. This is fatal for UP machine.

There is no good way to fix this misbehavior but to kludge it. The
emulation has interrupt throttling register aka ITR which limits
interrupt rate and allows the guest to proceed this phase.
There is no problem with this kludge for Linux guests - it adjust the
value of it itself.

On the other hand according to the initial research in
    commit e9845f0985f088dd01790f4821026df0afba5795
    Author: Vincenzo Maffione <v.maffione@gmail.com>
    Date:   Fri Aug 2 18:30:52 2013 +0200

    e1000: add interrupt mitigation support

    ...

    Interrupt mitigation boosts performance when the guest suffers from
    an high interrupt rate (i.e. receiving short UDP packets at high packet
    rate). For some numerical results see the following link
    http://info.iet.unipi.it/~luigi/papers/20130520-rizzo-vm.pdf

this should also boost performance a bit.

See https://bugzilla.redhat.com/show_bug.cgi?id=874406 for additional
details.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Vincenzo Maffione <v.maffione@gmail.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
