irqchip/gic-v3-its: Make its_send_vinvall operate on a single ITS

Currently, its_send_vinvall operates on all ITSs. As we're about
to try and limit the amount of commands we send to ITSs that are
not involved in dealing with a given VM, let's redefine that
primitive so that it takes a target ITS as a parameter.

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