* add vboxDriver object to serve as a singleton global object that
  holds references to IVirtualBox and ISession to be shared among
  multiple connections. The vbox_driver is instantiated only once in
  the first call vboxGetDriverConnection function that is guarded by
  a mutex.

* call vbox API initialize only when the first connection is
  established, and likewise uninitialize when last connection
  disconnects. The prevents each subsequent connection from overwriting
  IVirtualBox/ISession instances of any other active connection that
  led to libvirtd segfaults. The virConnectOpen and virConnectClose
  implementations are guarded by mutex on the global vbox_driver_lock
  where the global vbox_driver object counts connectios and decides
  when it's safe to call vbox's init/uninit routines.

* add IVirutalBoxClient to vboxDriver and use it to in tandem with newer
  pfnClientInitialize/pfnClientUninitalize APIs for vbox versions that
  support it, to avoid usage of the old pfnComInitialize/Uninitialize.