xen/arm: Let the toolstack configure the number of SPIs

xen/arm: Let the toolstack configure the number of SPIs

Each domain may have a different number of IRQs depending on the devices
assigned to it.

Rather than re-using the number of IRQs used by the hardwared GIC, let
the toolstack specify the number of SPIs when the domain is created.
This will avoid wasting memory.

To calculate the number of SPIs, we take advantage of the fact that the
libxl interface can only expose 1:1 mapping and look for the largest SPI
in the list.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Wei Liu <wei.liu2@citrix.com>