char: Use error_printf() to print help and such

char: Use error_printf() to print help and such

Calling error_report() in a function that takes an Error ** argument
is suspicious.  Convert a few that are actually help and such to
error_printf().

Improves output of -chardev help from

    qemu-system-x86_64: -chardev help: Available chardev backend types:
    serial
    ...

to

    Available chardev backend types:
    serial
    ...

Cc: Paolo Bonzini <[email protected]>
Cc: "Marc-André Lureau" <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20181017082702[email protected]>