arm: use new irqchip parameter to create different vGIC types

Currently we unconditionally create a virtual GICv2 in the guest.
Add a --irqchip= parameter to let the user specify a different GIC
type for the guest, when omitting this parameter it still defaults to
--irqchip=gicv2.
For now the only other supported type is --irqchip=gicv3

Signed-off-by: Andre Przywara <andre.przywara@arm.com>