ui: drop VNC feature _MASK constants

ui: drop VNC feature _MASK constants

Each VNC feature enum entry has a corresponding _MASK constant
which is the bit-shifted value. It is very easy for contributors
to accidentally use the _MASK constant, instead of the non-_MASK
constant, or the reverse. No compiler warning is possible and
it'll just silently do the wrong thing at runtime.

By introducing the vnc_set_feature helper method, we can drop
all the _MASK constants and thus prevent any future accidents.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>