xen/arm: vgic: Introduce a function to initialize pending_irq

xen/arm: vgic: Introduce a function to initialize pending_irq

The structure pending_irq is initialized in the same way in 2 different
places. Introduce vgic_init_pending_irq to avoid code duplication.

Also move the setting of the irq field into this function as we need to
initialize it once rather than every time an IRQ is injected to the guest.

Finally, use unsigned int for the "irq" field to be consistent with the
virq variable

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>