Re: [libvirt] [PATCH 3/4] vbox: change API (un)initialization logic.

Since VBOX API initialization method (pfnComInitialize) is not thread-safe and must be called from the primary thread, calling it in every vboxConnectOpen (as we used to do) leads to segmentation faults when multiple connections are in use. Therefore the initalization and unitialization logic has been changed as the following: * _registerGlobalData allocates g_pVBoxGlobalData (only when not already allocated) and calls VBOX's pfnComInitialize to grab references to ISession and IVirtualBox objects. This ensures that pfnComInitialize is called when the first connection is established.

* _pfnInitialize sets up the virConnectPtr->privateData (aka vboxPrivateData) for each connection by copying references to ISession and IVirtualBox from g_pVBoxGlobalData so that the rest of the driver API can use it without referencing the global. Each time this happens, a conntionCount is incremented on g_pVBoxGlobalData to keep track of when it's safe to call pfnComUnitialize. One of the reasons for existence of per-connection vboxPrivateData rather than completely relying on vboxGlobalData, is that more modern VBOX APIs provide pfnClientInitialize (since 4.2.20 and 4.3.4) and pfnClientThreadInitialize (5.0+) that allow to create multiple instances of ISession so if the code ever gets ported to support those newer APIs it should create much less diff noise as all API implementations are already updated to assume per-connection ISession/IVirutalBox instances. * _pfnUnitilalize decrements connectionCount in g_pVBoxGlobalData and once it is down to 0, it calls pfnComUnitialize and g_pVBoxGlobalData if free'd. This ensures that pfnComInitialize and pfnComUnitialize pair is called only once, even when multiple concurrent connections are in use.