arm/libxl: Emulated PCI device tree node in libxl

arm/libxl: Emulated PCI device tree node in libxl

libxl will create an emulated PCI device tree node in the device tree to
enable the guest OS to discover the virtual PCI during guest boot.
Emulated PCI device tree node will only be created when there is any
device assigned to guest.

A new area has been reserved in the arm guest physical map at
which the VPCI bus is declared in the device tree (reg and ranges
parameters of the node).

Note that currently we are using num_pcidevs instead of
c_info->passthrough to decide whether to create a vPCI DT node.
This will be insufficient if and when ARM does PCI hotplug.
Add this note inside libxl_create.c where c_info->passthrough
is set.

Signed-off-by: Rahul Singh <rahul.singh@arm.com>
Signed-off-by: Michal Orzel <michal.orzel@arm.com>
Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Acked-by: Ian Jackson <iwj@xenproject.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>