arm/xen: Correctly check if the event channel interrupt is present
arm/xen: Correctly check if the event channel interrupt is present

The function irq_of_parse_and_map returns 0 when the IRQ is not found.

Futhermore, move the check before notifying the user that we are running on
Xen.

Signed-off-by: Julien Grall <2fe41d4e32b0bb991f63b975f2bed08c1e666665@linaro.org>
Signed-off-by: Stefano Stabellini <22a0c6385d750d07aa383c9f813cf6f0228657d8@eu.citrix.com>
Acked-by: Ian Campbell <9be92b3dbbd6611e3e4c9209fb3d04b4919e7cca@citrix.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs