kvm: Rename irqchip_inject_ioctl to irq_set_ioctl

This variable is no longer bound to irqchip, and the IOCTL sets the IRQ
level, does not directly inject it. No functional changes.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>