VirtIO: Fix QEMU crash during Windows PNP tests

Hello,

In some cases bus driver can deassert "bus master" bit in PCI command
register. The driver will no longer be able to update related registers in
the device. Eventually it will cause QEMU to exit in "virtqueue_num_heads"
function.

Attached path that fixes the described issue.

Best regards,
Yan Vugenfirer.

>From 3fdafbdfad676ec8479dc073cff70bf356868bfe Mon Sep 17 00:00:00 2001
From: Yan Vugenfirer <yvugenfi@redhat.com>
Date: Tue, 8 Sep 2009 10:08:14 -0400
Subject: [PATCH] VirtIO: Fix QEMU crash during Windows PNP tests

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
