libxl: add generic function to add device

libxl: add generic function to add device

Add libxl__device_add to simple write XenStore device conifg
and libxl__device_add_async to update domain configuration
and write XenStore device config asynchroniously.
Almost all devices have similar libxl__device_xxxx_add function.
This generic functions implement same functionality but
using the device handling framework. Th device specific
part such as setting xen store configurationis moved
to set_xenstore_config callback of the device framework.

Signed-off-by: Oleksandr Grytsov <oleksandr_grytsov@epam.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>