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 <pbonzini@redhat.com>
Cc: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20181017082702.5581-8-armbru@redhat.com>