Change fw_cfg_add_file() to get full file path as a parameter instead
of building one internally. Two reasons for that. First caller may need
to know how file is named. Second this moves policy of file naming out
from fw_cfg. Platform may want to use more then two levels of
directories for instance.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>