blockdev: Make -drive format=help print to stdout

Command line help explicitly requested by the user should be printed
to stdout, not stderr.  We do elsewhere.  Adjust -drive to match: use
qemu_printf() instead of error_printf().  Plain printf() would be
wrong because we need to print to the current monitor for "drive_add
... format=help".

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190417190641.26814-13-armbru@redhat.com>