ARM: new VGIC: Handle virtual IRQ allocation/reservation

ARM: new VGIC: Handle virtual IRQ allocation/reservation

To find an unused virtual IRQ number Xen uses a scheme to track used
virtual IRQs.
Implement this interface in the new VGIC to make the Xen core/arch code
happy.
This is actually somewhat VGIC agnostic, so is mostly a copy of the code
from the old VGIC. But it has to live in the VGIC files, so we can't
easily reuse the existing implementation.

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>