hw/intc/arm_gic_kvm: Don't assume kernel can provide a GICv2

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:

qemu-system-aarch64: PMU: KVM_SET_DEVICE_ATTR: Invalid argument
qemu-system-aarch64: failed to set irq for PMU
Aborted

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.

With this patch then the user misconfiguration which previously
caused an abort now prints:
qemu-system-aarch64: Initialization of device kvm-arm-gic failed: error creating in-kernel VGIC: No such device
Perhaps the host CPU does not support GICv2?

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Tested-by: Andrew Jones <drjones@redhat.com>
Message-id: 20200225182435.1131-1-peter.maydell@linaro.org

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