hw/intc/arm_gic_kvm: Don't assume kernel can provide a GICv2handle-gicv3-only

In our KVM GICv2 realize function, we try to cope with old kernels
that don't provide the device control API (KVM_CAP_DEVICE_CTRL): we
try to use the device control, and if that fails we fall back to
assuming that the kernel has the old style KVM_CREATE_IRQCHIP and
that it will provide a GICv2.

This doesn't cater for the possibility of a kernel and hardware which
only provide a GICv3, which is very common now.  On that setup we
will abort() later on in kvm_arm_pmu_set_irq() when we try to wire up
an interrupt to the GIC we failed to create.

If the kernel advertises KVM_CAP_DEVICE_CTRL we should trust it if it
says it can't create a GICv2, rather than assuming it has one.  We
can then produce a more helpful error message including a hint about
the most probable reason for the failure.

If the kernel doesn't advertise KVM_CAP_DEVICE_CTRL then it is truly
ancient by this point but we might as well still fall back to a
KVM_CREATE_IRQCHIP GICv2.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

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