hw/arm/virt: kvm: allow gicv3 by default if v2 cannot work

hw/arm/virt: kvm: allow gicv3 by default if v2 cannot work

At the moment if the end-user does not specify the gic-version along
with KVM acceleration, v2 is set by default. However most of the
systems now have GICv3 and sometimes they do not support GICv2
compatibility.

This patch keeps the default v2 selection in all cases except
in the KVM accelerated mode when either
- the host does not support GICv2 in-kernel emulation or
- number of VCPUS exceeds 8.

Those cases did not work anyway so we do not break any compatibility.
Now we get v3 selected in such a case.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-id: 20200311131618.7187-7-eric.auger@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>