Blindly setting FD_CLOEXEC without a read-modify-write will
inadvertently clear any other intentionally-set bits, such as a
proposed new bit for designating a fd that must behave in 32-bit mode.
However, we cannot use our wrapper qemu_set_cloexec(), because that
wrapper intentionally abort()s on failure, whereas the probe here
intentionally tolerates failure to deal with incorrect socket
activation gracefully.  Instead, fix the code to do the proper
read-modify-write.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200420175309.75894-3-eblake@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>