linux-user: Consistently return host errnos from do_openat()

linux-user: Consistently return host errnos from do_openat()

The function do_openat() is not consistent about whether it is
returning a host errno or a guest errno in case of failure.
Standardise on returning -1 with errno set (ie caller has
to call get_errno()).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reported-by: Timothy Edward Baldwin <T.E.Baldwin99@members.leeds.ac.uk>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>