vfs: Fix isatty on console file descriptors

file descriptors 0, 1 and 2 aren't vfs files,
thus isatty cannot rely on a vnode to determine
whether or not the device underlying the file
is tty.

Avi suggested adding a new file subclass and using
dynamic_cast to avoid introcucing a new file type
to specifically address this problem.

According to Gilad, this bug was preventing lua
from starting in interactive mode.

Reported-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>