util, conf: Handle default monitor group of an allocation properly

util, conf: Handle default monitor group of an allocation properly

'default monitor of an allocation' is defined as the resctrl
monitor group that created along with an resctrl allocation,
which is created by resctrl file system. If the monitor group
specified in domain configuration file is happened to be a
default monitor group of an allocation, then it is not necessary
to create monitor group since it is already created. But if
an monitor group is not an allocation default group, you
should create the group under folder
'/sys/fs/resctrl/mon_groups' and fill the vcpu PIDs to 'tasks'
file.

Signed-off-by: Wang Huaqiang <huaqiang.wang@intel.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>