arm/arm64: KVM: vgic: make number of irqs a configurable attribute

arm/arm64: KVM: vgic: make number of irqs a configurable attribute

In order to make the number of interrupts configurable, use the new
fancy device management API to add KVM_DEV_ARM_VGIC_GRP_NR_IRQS as
a VGIC configurable attribute.

Userspace can now specify the exact size of the GIC (by increments
of 32 interrupts).

Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
(cherry picked from commit a98f26f183801685ef57333de4bafd4bbc692c7c)
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>