fiwalk: Allow plugins to process some virtual files

A boot-sector virus was not being detected by a Fiwalk plugin.  The
virus was detected when directly scanning the disk image, and when
scanning the boot sector extracted from the image.

This patch allows Fiwalk to pass a specific virtual file to plugin
processes.

Reported-by: Christie Peterson <cpeterson@jhu.edu>
Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>