Move fcntl_setfl() to oslib-posix

It is only implemented for POSIX anyway.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220323155743.1585078-30-marcandre.lureau@redhat.com>
[Add braces around if statements. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>