piix4acpi, xen: Clarify that the qemu_set_irq calls just do an IRQ pulse.

piix4acpi, xen: Clarify that the qemu_set_irq calls just do an IRQ pulse.

The "qemu_cpu_notify" raises and lowers the ACPI SCI line when the
vCPU state has changed.

Instead of doing the two functions, just use one function that
describes exactly what it does.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com> (for 4.3 release)