ARM: KVM: vgic: force EOIed LRs to the empty state

The VGIC doesn't guarantee that an EOIed LR that has been configured
to generate a maintenance interrupt will appear as empty.

While the code recovers from this situation, it is better to clean
the LR and flag it as empty so it can be quickly recycled.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>