hw/char/spapr_vty: Return amount of free buffer entries in vty_can_receive()

The can_receive() callbacks of the character devices should return
the amount of characters that can be accepted at once, not just a
boolean value (which rather means only one character at a time).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

hw/char/spapr_vty.c

@@ -25,7 +25,7 @@ static int vty_can_receive(void *opaque)
 {
     VIOsPAPRVTYDevice *dev = VIO_SPAPR_VTY_DEVICE(opaque);
 
-    return (dev->in - dev->out) < VTERM_BUFSIZE;
+    return VTERM_BUFSIZE - (dev->in - dev->out);
 }
 
 static void vty_receive(void *opaque, const uint8_t *buf, int size)