q35: Revert to kernel irqchip

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")
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <155786484688.13873.6037015630912983760.stgit@gimli.home>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>