xen/arm: route_irq_to_guest: Check validity of the IRQ

xen/arm: route_irq_to_guest: Check validity of the IRQ

Currently Xen only supports SPIs routing for guest, add a function
is_assignable_irq to check if we can assign a given IRQ to the guest.

Secondly, make sure the vIRQ is not the greater than the number of IRQs
configured in the vGIC and it's an SPI.

Thirdly, when the IRQ is already assigned to the domain, check the user
is not asking to use a different vIRQ than the one already bound.

Finally, desc->arch.type which contains the IRQ type (i.e level/edge) must
be correctly configured before. The misconfiguration can happen when:
    - the device has been blacklisted for the current platform
    - the IRQ has not been described in the device tree

Also, use XENLOG_G_ERR in the error message within the function as it will
be later called from a guest.

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>