kvm tools: Allow piping debug output to file descriptor

This patch makes debug output go to a 'debug_fd' instead of stdout.

Doing so allows us to send the output to a different console when
required.

This patch also changes the behaviour of 'kvm debug' to show the debug
output in the console that executed the debug command instead of in the
console of the guest.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>