Otherwise this beautiful error would be overwritten when
the function is called with a really high rate number:

2014-07-28 12:51:47.920+0000: 2304: error : virCommandWait:2399 :
internal error: Child process (/sbin/tc class add dev vnet0 parent 1:
classid 1:1 htb rate 4294968kbps) unexpected exit status 1: Illegal "rate"
Usage: ... qdisc add ... htb [default N] [r2q N]
 default  minor id of class to which unclassified packets are sent {0}
 r2q      DRR quantums are computed as rate in Bps/r2q {10}
 debug    string of 16 numbers each 0-3 {0}

... class add ... htb rate R1 [burst B1] [mpu B] [overhead O]
                      [prio P] [slot S] [pslot PS]
                      [ceil R2] [cburst B2] [mtu MTU] [quantum Q]
 rate     rate allocated to this class (class can still borrow)
 burst    max bytes burst which can be accumulated during idle period {computed}
 mpu      minimum packet size used in rate computations
 overhead per-packet size overhead used in rate computations
 linklay  adapting to a linklayer e.g. atm
 ceil     definite upper class rate (no borrows) {rate}
 cburst   burst but for ceil {computed}
 mtu      max packet size we create rate map for {1600}
 prio     priority of leaf; lowe

https://bugzilla.redhat.com/show_bug.cgi?id=1043735