vv-file: Add VirtViewerFile::versions
    
    Specifying a single minimal version in the .vv file is not enough as
    the client version will be highly dependent on the OS it's running in.
    Windows versioning is not the same as linux versioning, Fedora 21 and
    Fedora 22 may have different release numbers for the same version,
    and we may want to force a specific minimal release in case of a
    critical bug fix.
    
    This commit adds supports for a 'versions' field in .vv files where a
    list of os-id:version couples can be stored.