code跟踪函数时，遇到函数指针，该怎么继续跟踪

示例：vgaR3IOPortHGSMIWrite

1. PDMDevHlpIOPortRegister（vgaR3IOPortHGSMIWrite），跟踪PDMDevHlpIOPortRegister（）

DECLINLINE(int) PDMDevHlpIOPortRegister(PPDMDEVINS pDevIns, RTIOPORT Port, RTIOPORT cPorts, RTHCPTR pvUser,
                                        PFNIOMIOPORTOUT pfnOut, PFNIOMIOPORTIN pfnIn,
                                        PFNIOMIOPORTOUTSTRING pfnOutStr, PFNIOMIOPORTINSTRING pfnInStr, const char *pszDesc)
{
    return pDevIns->pHlpR3->pfnIOPortRegister(pDevIns, Port, cPorts, pvUser, pfnOut, pfnIn, pfnOutStr, pfnInStr, pszDesc);
}

2. pfnIOPortRegister(pfnOut)
    pfnIOPortRegister(vgaR3IOPortHGSMIWrite)

    这里不再跟踪pfnIOPortRegister, 而是跟踪pfnOut的参数类型 PFNIOMIOPORTOUT
    vgaR3IOPortHGSMIWrite肯定是赋值给xxx变量，xxx变量的参数类型也一定是 PFNIOMIOPORTOUT

3. 接下来找出的八成是一个结构体
找到pfnOutCallback变量，变量类型为 PFNIOMIOPORTOUT
pfnOutCallback被vgaR3IOPortHGSMIWrite赋值后，肯定就当作一个函数使用，跟踪pfnOutCallback