util/guest-random: Fix size arg to tail memcpy

util/guest-random: Fix size arg to tail memcpy

We know that in the body of this if statement i is less than len, so
we really should be copying len - i bytes not i - len bytes.

Fix this typo.

Fixes: 8d8404f1564 ("util: Add qemu_guest_getrandom and associated routines")
Signed-off-by: Mark Nelson <mdnelson8@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210709120600.11080-1-mdnelson8@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>