apibuild: Substitute only pure number tokens
    
    In 38df47c9af1 I've tried to prepare our apibuild.py script for
    change made in 0628f3498ce (1U << 31). What I've done in the
    former commit was to replace \d+U in parsed tokens with \d.
    Problem was, my regular expression there was not quite right as
    it also translated VIR_123U_VAL into VIR_123_VAL.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>