linux-user: introduce preexit_cleanup

linux-user: introduce preexit_cleanup

To avoid repeating ourselves move our preexit clean-up code into a
helper function. I figured the continuing effort to split of the
syscalls made it worthwhile creating a new file for it now.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>