Check the return value of fcntl in qemu_set_cloexec

Assert that the return value is not an error. This issue was found by
Coverity.

CID: 1374831

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
CC: groug@kaod.org
CC: pbonzini@redhat.com
CC: Eric Blake <eblake@redhat.com>
Message-Id: <1494356693-13190-2-git-send-email-sstabellini@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
