allow MSI with the QEMU default pc-i440fx machine

allow MSI with the QEMU default pc-i440fx machine

This makes it possible to use MSI for virtual functions of Intel network
devices without having to specify the q35 machine.

QEMU is detected by testing for the Qumranet pci subsystem vendor id,
suggested by sf@.  MSI previously wasn't enabled as i440fx models a machine
from 1996 with ACPI 1.0, and MSI is only enabled for ACPI >= 2.0.

Initial patch from Yuichiro NAITO. ok yasuoka@ sf@