fs: clean up old "fo_*" C functions

Instead of the old C-style file-operation function types and fo_*()
functions, since recently we have methods of the "file" class. All our
filesystem code is now C++, and can use these methods directly.

So this patch drops the old types and functions, and uses the class methods
instead.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>