qemu.balloon_check: reset the memory to origin value

The memory should be reset to the origin value after
evict or enlarge. So that the next iterations tests can
pass when check memory before test

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>
Merge pull request #641 from QiuMike/qemu.balloon_check_reset_the_memory_to_origin_value

qemu.balloon_check: reset the memory to origin value