Add version check for extensions to logs

[Will Everett]

For linux distros that include virtualbox updates with the system, it's really easy for version mismatches to occur between the main program and your extensions. Sometimes this causes obvious problems and sometimes it doesn't so something in the boot logs that just lists the versions of your extensions all in one place and maybe says something like "Warning: extension versions don't match VirtualBox version" would be helpful for troubleshooting some kinds of sudden breaks.

[Frank Mehnert]

Sorry, but

1. That information is available in the VBox.log file and
2. VBox will automatically warn if the Extension Pack does not fit to the base package and offer to download the correct package from the internet.

I know that many distributions disable that logic but then I don't see why we should implement another logic for such cases.