Use the utility routine qemu_set_cloexec() rather than
manually calling fcntl(). This lets us drop the #ifndef _WIN32
guards and also means Coverity doesn't complain that we're
ignoring the fcntl error return (CID 1005665, CID 1005667).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>