Allow users to provide custom fw_cfg blobs with ascii string
payloads specified directly on the qemu command line.

Suggested-by: Jordan Justen <jordan.l.justen@intel.com>
Suggested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Message-id: 1443544141-26568-1-git-send-email-somlo@cmu.edu
Reviewd-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>