qemu.balloon_check:add the missing command to cfg file

When run the case balloon_check.balloon_evict.balloon-migrate,
it will get 'SKIP' result becasue the needed command
'migration_test_command' has been removed.This command
will be used in migration tests.

Readd the command to solve this problem.

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>