用法：libvirt = __import__('libvirt')
help(__import__)
__import__(...)
__import__(name, globals={}, locals={}, fromlist=[], level=-1) -> module
导入一个模块。
globals：决定上下文，不修改。
locals：当前未使用。
fromlist：有值表示``from name import ...''，为空表示``import name''。
当从一个包中导入模块时，注意__import__('A.B', ...)当fromlist为空时返回包A，
不为空时返回子模块B。
level：用于决定是否执行绝对导入。-1是绝对相对都会尝试，默认。0是绝对。正数
表示相对当前模块的父目录的层数。
Import a module. The globals are only used to determine the context;
they are not modified. The locals are currently unused. The fromlist
should be a list of names to emulate ``from name import ...'', or an
empty list to emulate ``import name''.
When importing a module from a package, note that __import__('A.B', ...)
returns package A when fromlist is empty, but its submodule B when
fromlist is not empty. Level is used to determine whether to perform
absolute or relative imports. -1 is the original strategy of attempting
both absolute and relative imports, 0 is absolute, a positive number
is the number of parent directories to search relative to the current module.