vnc: Clean up error reporting in vnc_init_func()

vnc: Clean up error reporting in vnc_init_func()

Calling error_report() in a function that takes an Error ** argument
is suspicious.  vnc_init_func() does that, and then fails without
setting an error.  Its caller main(), via qemu_opts_foreach(), is fine
with it, but clean it up anyway.

While there, drop a "Failed to start VNC server: " error message
prefix that doesn't really add value.

Cc: Gerd Hoffmann <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20181017082702[email protected]>
Reviewed-by: Gerd Hoffmann <[email protected]>