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 <kraxel@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20181017082702.5581-28-armbru@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>