vbox: Avoid signed and unsigned comparison
    
    After 457ff97fa there are two defects in our code. In both of
    them we use a signed variable to hold up a number of snapshots
    that domain has. We use a helper function to count the number.
    However, the helper function may fail in which case it returns
    a negative one and control jumps to cleanup label where an
    unsigned variable is used to iterate over array of snapshots. The
    loop condition thus compare signed and unsigned variables which
    in this specific case ends up badly for us.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>