vnc: improve capslock handling.

When capslock is toggled while the vnc window hasn't the focus qemu
will miss the state change.  Add sanity checks for the capslock state
and toggle it if needed, so hosts and guests idea of capslock state
stay in sync.  Simliar logic for numlock is present in qemu already.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
