qemuSetupCgroupVcpuBW() and lxcSetVcpuBWLive() shares the
same code to set CPU CFS period and quota. This code can be
moved to a new virCgroupSetupCpuPeriodQuota() helper to
avoid code repetition.

A similar code is also executed in virLXCCgroupSetupCpuTune(),
but without the rollback on error. Use the new helper in this
function as well since the 'period' rollback, if not a
straight improvement for virLXCCgroupSetupCpuTune(), is
benign. And we end up cutting more code repetition.
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>