libxl: Add support for Device Tree passthrough

libxl: Add support for Device Tree passthrough

On ARM, every non-PCI device are described in the device tree. Each of
them can be found via a path.

This patch introduces a very basic support, only the IOMMU will be set
up correctly. The user will have to:
    - Describe the device in the partial device tree
    - Map manually MMIO/IRQ

This is a first approach, that will allow to have a basic Device Tree
passthrough support in Xen. This could be improved later.

Furthermore add LIBXL_HAVE_DEVICETREE_PASSTHROUGH to indicate we
support Device Tree passthrough and partial device tree (introduced by a
previous patch).

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