kvm tools: Rework stdio/stdout handling to support redirection

Currently if you redirect the output from "lkvm run" to a file then
term_init() will fail, because it can't call the terminal ioctls.

So check if stdin and stdout are ttys, if either is not then skip the
rest of the terminal setup. Redirecting one but not the other is a
little odd, but does work.

Note that we skip registering the cleanup routines, so we don't need to
modify them.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Pekka Enberg <penberg@kernel.org>