io vnc sockets: Clean up SocketAddressKind switches

io vnc sockets: Clean up SocketAddressKind switches

We have quite a few switches over SocketAddressKind.  Some have case
labels for all enumeration values, others rely on a default label.
Some abort when the value isn't a valid SocketAddressKind, others
report an error then.

Unify as follows.  Always provide case labels for all enumeration
values, to clarify intent.  Abort when the value isn't a valid
SocketAddressKind, because the program state is messed up then.

Improve a few error messages while there.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-id: 1490895797[email protected]
Signed-off-by: Max Reitz <[email protected]>