If no unar on Linux, use 7z

On some distributions, like Fedora, `unar` is not readily available from a repository. However `7za`, which is generally available from the `p7zip` package (e.g. http://koji.fedoraproject.org/koji/packageinfo?packageID=2715) is available, and works fine to unzip the large VM files.

Modified the script to check for and use `7za` if `unar` is not available.