Example launcher handle guests not being ready for input

We currently discard console and network input when the guest has no
input buffers.  This patch changes that, so that we simply stop
listening to that fd until the guest refills its input buffers.

This is particularly important because hvc_console without interrupts
does backoff polling and so often lose characters if we discard.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>