virsh: Implement VIR_DOMAIN_BANDWIDTH_IN_FLOOR
    
    We have a function parseRateStr() that parses --inbound and
    --outbound arguments to both attach-interface and domiftune.
    Now that we have all virTypedParams macros needed for QoS,
    lets parse even floor attribute. The extended format for the
    arguments looks like this then:
    
      --inbound average[,peak[,burst[,floor]]]
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>