Fixes logic for calculating percentage for cgroups.Stats.CpuStats.CpuUsage.PercentUsage
Previous logic divided a uint64 by a uint64 losing precision before multiplying by clock ticks and cpus
This fix maintains precision before converting back to uint64

Docker-DCO-1.1-Signed-off-by: Nicholas Weaver <lynxbat@gmail.com> (github: lynxbat)
Merge branch 'master' of https://github.com/docker/libcontainer into 145-PercentUsage
Removed an extra float64()

Docker-DCO-1.1-Signed-off-by: Nicholas Weaver <lynxbat@gmail.com> (github: lynxbat)
Merge pull request #147 from lynxbat/145-PercentUsage

Fixes logic for calculating percentage