libxl: build a device tree for ARM guests

libxl: build a device tree for ARM guests

Uses xc_dom_devicetree_mem which was just added. The call to this needs to be
carefully sequenced to be after xc_dom_parse_image (so we can tell which kind
of guest we are building, although we don't use this yet) and before
xc_dom_mem_init which tries to decide where to place the FDT in guest RAM.

Removes libxl_noarch which would only have been used by IA64 after this
change. Remove IA64 as part of this patch.

There is no attempt to expose this as a configuration setting for the user.

Includes a debug hook to dump the dtb to a file for inspection.

On v7 the CPU compat is currently hardcoded to cortex-a15. This likely wants
to change at some point.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>