The call of virResctrlMonitorGetStats will allocate the memory for
holding cache occupancy or memory bandwidth statistics.

This patch adds the function virResctrlMonitorFreeStats as the
opposing action of virResctrlMonitorGetStats to free the memory.
Signed-off-by: NWang Huaqiang <huaqiang.wang@intel.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>