update-guest-additions: allow version to be specified as command-line argument

As a fallback to automatic host VirtualBox version detection, we should allow
the user to specify the version as a command-line argument to
'update-guest-additions'. This patch implements this functionality. The
argument is '--virtualbox-version'.

Change-Id: I3b4a4f6c422364a0d903d2a2b1379719c7ed0e43