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>