Allow user supplied files to be inserted into the fw_cfg
device before starting the guest. Since fw_cfg_add_file()
already disallows duplicate fw_cfg file names, qemu will
exit with an error message if the user supplies multiple
blobs with the same fw_cfg file name, or if a blob name
collides with a fw_cfg name programmatically added from
within the QEMU source code. A warning message will be
printed if the fw_cfg item name does not begin with the
prefix "opt/", which is recommended for external, user
provided blobs.