Refactor schemas and virresctrl to support optional <cache> element
in <cachetune>.

Later, the monitor entry will be introduced and to be placed
under <cachetune>. Either cache entry or monitor entry is
an optional element of <cachetune>.

An cachetune has no <cache> element is taking the default resource
allocating policy defined in '/sys/fs/resctrl/schemata'.
Signed-off-by: NWang Huaqiang <huaqiang.wang@intel.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>