At the moment kvmtool always tries to instantiate a virtual GICv2
interrupt controller for the guest, and fails with some scary error
message if that doesn't work.
The user has then to manually specify "--irqchip=gicv3", which is not
really obvious.
With the advent of more GICv3-only machines, let's try to be more
clever and implement some auto-detection of the GIC type needed:
We try gicv3-its, gicv3, gicv2m and gicv2, in that order. The first one
succeeding wins.
For GICv2 machines the first two will always fail.
On GICv3 machines offering GICv2 compatibility we used to prefer a
virtual GICv2 in the guest, but these days the GICv3 support both in
guests and in KVM is equally mature and wide-spread, so we should use
the GICv3 emulation for the guest as well.

This algorithm is in effect is there is no explicit --irqchip parameter
on the command line. We still allow the GIC type to be set explicitly.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>