tests/qtest/libqos: Add generic pci host bridge in arm-virt machine

tests/qtest/libqos: Add generic pci host bridge in arm-virt machine

Up to now the virt-machine node contains a virtio-mmio node.
However no driver produces any PCI interface node. Hence, PCI
tests cannot be run with aarch64 binary.

Add a GPEX driver node that produces a pci interface node. This latter
then can be consumed by all the pci tests. One of the first motivation
was to be able to run the virtio-iommu-pci tests.

We still face an issue with pci hotplug tests as hotplug cannot happen
on the pcie root bus and require a generic root port. This will be
addressed later on.

We force cpu=max along with aarch64/virt machine as some PCI tests
require high MMIO regions to be available.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Message-Id: <20220504152025.1785704-4-eric.auger@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>