qemu-io-cmds: Fix two format strings

Use %zu instead of %zd for unsigned numbers.

This fixes two error messages from the LSTM static code analyzer:

    This argument should be of type 'ssize_t' but is of type 'unsigned long'

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>