baum: Truncate braille device size to 84x1

Baum device bigger than 84 do not actually exist, but the user's own
Braille device might be wider than 84 columns.  Some guest drivers
would be upset by such sizes, so clamp the device size.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Message-Id: <20171211001950.27843-1-samuel.thibault@ens-lyon.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>