KVM: arm/arm64: GICv4: Handle CLEAR applied to a VLPI

Handling CLEAR is pretty easy. Just ask the ITS driver to clear
the corresponding pending bit (which will turn into a CLEAR
command on the physical side).

Acked-by: Christoffer Dall <cdall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>