all: Allow backend to provide a better get_config_descriptor_by_value

all: Allow backend to provide a better get_config_descriptor_by_value

Our core get_config_descriptor_by_value is not exactly pretty nor efficient,
allow the backends to provide something better.

Note that the callback signature differs from get_config_descriptor in that
backend owned memory gets returned. This saves a needless malloc + memcpy +
free. If this turns out to be a problem for some backends we can always
change things to work like get_config_descriptor.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>