virtio: console: make get_inbuf() return port->inbuf if present

Instead of pulling in a buffer from the vq each time it's called,
get_inbuf() now checks if the current active buffer, in port->inbuf is
valid.  If it is, just returns a pointer to it.  This ends up
simplifying a lot of code calling get_inbuf() since the check for
port->inbuf being valid was done by all the callers.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>