sim: allow memory maps to default to mapped files

sim: allow memory maps to default to mapped files

I find it annoying when using --memory-mapfile that I also need to look
up and manually specify the file size to the following --memory-region
option.  So make a length of 0 in the following --memory-region trigger
an auto-sizing of the map to the length of the file being mapped.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>