fix cgroup_get_value_string problem with *.stat output

fix cgroup_get_value_string problem with *.stat output

fix cgroup_get_value_string problem with *.stat output
the problem was in the low limit for value for the output of stat variables - now there are used the functions cgroup_read_stats_{begin/next/end} for this purpose

bogus output:
./src/tools/cgget -r memory.stat /
/:
memory.stat=cache 1024561152
rss 291270656
pgpgin 1209259
pgpgout 888011
inactive_anon 8192
active_anon 2976481

fixed output:
$ ./src/tools/cgget -r memory.stat /
/:
memory.stat=cache 1024610304
	rss 291270656
	pgpgin 1209271
	pgpgout 888011
	inactive_anon 8192
	active_anon 297648128
	inactive_file 619126784
	active_file 399073280
	unevictable 24576
	hierarchical_memory_limit 9223372036854775807
	hierarchical_memsw_limit 9223372036854775807
	total_cache 1024610304
	total_rss 291270656
	total_pgpgin 1209271
	total_pgpgout 888011
	total_inactive_anon 8192
	total_active_anon 297648128
	total_inactive_file 619126784
	total_active_file 399073280
	total_unevictable 24576

Signed-off-by: Ivana Hutarova Varekova <varekova@redhat.com>
Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>