Xen and KVM are the only supported backends for now

[SiChuchen]

In the Xen environment, when running the example process example/hookguest, the error "Xen and KVM are the only supported backends for now" occurred. This issue not only affects the current example but also causes the daemon in the hvmi project(https://github.com/bitdefender/hvmi) to be non-functional.

I eventually found the source of the error in the file libbdvmi/src/backendfactory.cpp, where USE_XEN and USE_KVMI were used, but it seems that they were not defined elsewhere, causing a direct jump to the error message.

My current solution is quite simple: since I am only using the Xen environment, I defined USE_XEN at the beginning of that file, recompiled, and then both the example process and the daemon under hvmi were able to run successfully.

在 xen 环境下，在运行示例进程 example/hookguest 的时候出现 ：Xen and KVM are the only supported backends for now，该问题不仅影响当前示例，也导致 hvmi 项目中的 daemon 不可运行。
最终找到错误来源是 libbdvmi/src/backendfactory.cpp 该文件中使用 USE_XEN 和 USE_KVMI，但貌似在别的地方对其没有进行定义，致使直接跳转到错误中。
目前我的做法比较简单，由于我只使用 xen 环境，所以在该文件开头对 USE_XEN 进行定义，重新进行编译，然后示例进程和 hvmi 下的守护进程都可以运行了。