1. A server system, comprising: at least one CPU; a memory for storing data and a program; and an interface for exchanging data with the outside, wherein: the CPU executes the program stored in the memory to execute a virtual machine monitor for operating at least one virtual computer in which an operating system operates; and the virtual machine monitor includes: an emulation module for emulating an operation based on an operation code for executing the operation of hardware of the server system when the operation of the hardware of the server system is requested by one of the operating system and an application operated in the operating system; an exception event handler module for calling the emulation module when an exception event is generated by the CPU through detection of the operation of the hardware of the server system; a code management module for managing a promotion code used for emulating the operation of the hardware of the server system by switching an operation mode of the CPU to call the emulation module; a frequency judgment module for judging whether a frequency of the operation of the hardware of the server system higher than a predetermined threshold value based on a result of comparison with predetermined judgment conditions; and a switching module for determining whether to call the emulation module by one of the exception event handler module and by executing the promotion code based on the judged frequency, wherein the CPU includes two operation modes of a hardware operation permission mode and a hardware operation inhibition mode, and generates an exception event to switch the operation mode to the hardware operation permission mode when the operation of the hardware is detected while the operation mode is the hardware operation inhibition mode; the promotion code contains an instruction of switching between the hardware operation permission mode and the hardware operation inhibition mode; the virtual machine monitor includes a generation module for generating a shadow code to emulate the operation of the hardware of the server system, which corresponds to the operation code by calling the promotion code when the operation mode of the CPU is the hardware operation inhibition mode; when the operation is requested while the operation mode of the CPU is the hardware operation inhibition mode, the switching module executes the promotion code by calling the shadow code if a frequency of the operation of the hardware of the server system is higher than a predetermined threshold value, and calls the emulation module by the promotion code, the CPU is shifted to the hardware operation permission mode by setting a current privilege level to 0 and setting a VMX root mode, and shifted to the hardware operation inhibition mode by one of setting the current privilege level to a value other than 0 and setting a VMX non-root mode; the promotion code contains an instruction of changing the current privilege level; and the switching module is configured to: correlate an address of the operation code for executing the operation of the hardware of the server system with a storage address of the shadow code; and execute the shadow code by executing processing branching to the storage address of the shadow code in place of processing branching to the operation code based on the correlation when emulation of the operation is executed to call the emulation module by executing the shadow code.