virsh: Rework parseRateStr
    
    The function is used to parse a tuple delimited by commas into
    virNetDevBandwidth structure. So far only three out of fore
    fields are supported: average, peak and burst. The single missing
    field is floor. Well, the parsing works, but I think we can do
    better. Especially when we will need to parse floor too in very
    close future.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>