q35: Revert to kernel irqchip

q35: Revert to kernel irqchip

Backport of QEMU v4.1 commit for stable v4.0.1 release

commit c87759ce876a7a0b17c2bf4f0b964bd51f0ee871
Author: Alex Williamson <alex.williamson@redhat.com>
Date:   Tue May 14 14:14:41 2019 -0600

    q35: Revert to kernel irqchip

    Commit b2fc91db8447 ("q35: set split kernel irqchip as default") changed
    the default for the pc-q35-4.0 machine type to use split irqchip, which
    turned out to have disasterous effects on vfio-pci INTx support.  KVM
    resampling irqfds are registered for handling these interrupts, but
    these are non-functional in split irqchip mode.  We can't simply test
    for split irqchip in QEMU as userspace handling of this interrupt is a
    significant performance regression versus KVM handling (GeForce GPUs
    assigned to Windows VMs are non-functional without forcing MSI mode or
    re-enabling kernel irqchip).

    The resolution is to revert the change in default irqchip mode in the
    pc-q35-4.1 machine and create a pc-q35-4.0.1 machine for the 4.0-stable
    branch.  The qemu-q35-4.0 machine type should not be used in vfio-pci
    configurations for devices requiring legacy INTx support without
    explicitly modifying the VM configuration to use kernel irqchip.

Link: https://bugs.launchpad.net/qemu/+bug/1826422
Fixes: b2fc91db8447 ("q35: set split kernel irqchip as default")
Cc: qemu-stable@nongnu.org
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
(upstream commit c87759ce876a7a0b17c2bf4f0b964bd51f0ee871)
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
*add comments regarding AML mismatch warnings from
 tests/bios-tables-test.c
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>