arm/xen: Get event-channel irq through HVM_PARAM when booting with ACPI

The kernel will get the event-channel IRQ through
HVM_PARAM_CALLBACK_IRQ.

Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Tested-by: Julien Grall <julien.grall@arm.com>