file: ignore host disk in blocklist check

file: ignore host disk in blocklist check

It cannot work anyway because host disk cannot be read. This fixes hostfs access
on native Windows build where filenames start with '\' or do not have initial
separator at all (d:\foo).

Issue was observed when running grub-fstest on Windows. On UNIX image name is
canonicalized to always start with `/' so this was not noticed.

This has side effect of allowing relative path names on host, but this already
was the case with `ls' command, so it just extends it to all commands.

Reported-By: Arch Stack <archstacker@gmail.com>
Also-By: Arch Stack <archstacker@gmail.com>