libxl: remove list callback from device framework

libxl: remove list callback from device framework

As we have generic functions to get device list
(libxl__device_list) no need to have callback in
the framework. To resolve issue when XS entry
doesn't match device name, device type is
extended with field "entry" which keeps XS entry.

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