Fix VirtualBox version detection for Ubuntu builds

On Ubuntu, VirtualBox version strings have the form '4.2.10_Ubuntur8410' (for
example). Sanitizing that string by calling "ver.gsub(/r.*/m, '')" produces
"4.2.10_Ubuntu", which is incorrect. This change ensures the version number
extraction matches only numbers and the dot symbol.

Bug: 48467
Change-Id: Ib20e396ce1d3e40f86cab43a24ce3516daacb8a9