win32: cleanup VSSClient constructor

win32: cleanup VSSClient constructor

- VSSClient is a pure class that has virtual methods, using a
  memset() to initialize the class is not appropriate because the object
  include a VPTR that is initialized by the constructor
- gcc 11.3 complains about that
- initializing the class the right way cannot be wrong
- Using C++11 in-member class initializers (we are not building win32 on
  centos 5 anymore but on modern platforms :-)