vnc: rename vnc-encoding-* vnc-enc-*

For the same reason that we don't use vnc-authentication-sasl.c but
vnc-auth-sals.c. Because it's tooooo long.

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
