lsi: do not exit QEMU if reading invalid register

When guest accesses invalid register, return 0xff instead of exiting.
Also add a log when reading or writing invalid registers.

Signed-off-by: HervÃ© Poussineau <hpoussin@reactos.org>
Message-Id: <1471505489-1221-3-git-send-email-hpoussin@reactos.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
