Refactor cgroups internal data structures

Refactor cgroups internal data structures

Currently the virCgroupPtr struct contains 3 pieces of
information

 - path - path of the cgroup, relative to current process'
   cgroup placement
 - placement - current process' placement in each controller
 - mounts - mount point of each controller

When reading/writing cgroup settings, the path & placement
strings are combined to form the file path. This approach
only works if we assume all cgroups will be relative to
the current process' cgroup placement.

To allow support for managing cgroups at any place in the
heirarchy a change is needed. The 'placement' data should
reflect the absolute path to the cgroup, and the 'path'
value should no longer be used to form the paths to the
cgroup attribute files.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>