xen/passthrough: Extend XEN_DOMCTL_*assign_device to support DT device

xen/passthrough: Extend XEN_DOMCTL_*assign_device to support DT device

A device node is described by a path. It will be used to retrieve the
node in the device tree and assign the related device to the domain.

Only non-PCI devices protected by an IOMMU can be assigned to a guest.

Also document the behavior of XEN_DOMCTL_deassign_device in the public
headers which differ between non-PCI and PCI.

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