cgroups: Do not enforce nonexistent controllers

cgroups: Do not enforce nonexistent controllers

Currently, the controllers argument to virCgroupDetect acts both as
a result filter and a required controller specification, which is
a bit overloaded. If both functionalities are needed, it would be
better to have them seperated into a filter and a requirement mask.
The only situation where it is used today is to ensure that only
CPU related controllers are used for the VCPU directories. But here
we clearly do not want to enforce the existence of cpu, cpuacct and
specifically not cpuset at the same time.
This commit changes the semantics of controllers to "filter only".
Should a required mask ever be needed, more work will have to be done.

Signed-off-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>