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 <julien.grall@linaro.org>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
(cherry picked from commit 81e863c3a28e69fd60411bde9f779b0f8ad0212a)
Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>