Add ability to query version of VirtualBox in runtime

[Kentzo]

When developing extensions, there is a necessity to query version of VirtualBox (and versioned structures) in runtime. E.g. to provide different implementation for different versions without providing separate binary for each version of vbox.

API should provide static functions to query current version of any versioned entity.

[Frank Mehnert]

To detect the version of the runtime, use RTBldCfgRevision() / RTBldCfgVersionMajor() / RTBldCfgVersionMinor() / RTBldCfgVersionBuild(). And the API provides IVirtualBox::Version().