xen/arm: gic: Add sanity checks gic_route_irq_to_guest

xen/arm: gic: Add sanity checks gic_route_irq_to_guest

With the addition of interrupt assignment to guest, we need to make sure
the guest can't blow up the interrupt management in Xen.

Before associating the IRQ to a vIRQ we need to make sure:
    - the vIRQ is not already associated to another IRQ
    - the guest didn't enable the vIRQ

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>