cpu: improve cpu info virtualization

cpu: improve cpu info virtualization

This commit introduces several improvements to CPU views based on
quotas:
- fall back to cpuacct.usage_percpu if cpuacct.usage_all not exists
- correct CPU usage
- correct CPU usage in partial CPU cases when quota/period doesn't yield
  an integer

Signed-off-by: Stephen Xiang <BurningXFlame@gmail.com>
[christian.brauner@ubuntu.com: squashed commits and fixed up commit message]
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Abstract must_strcat from must_strcat_pid. Replace stringbuild with must_strcat.

Signed-off-by: Stephen Xiang <BurningXFlame@gmail.com>