Update control file for autotest.bonnie testing

Using default bonnie++ test command make testing always failed in 20G virtual disk image because of no enough free space to create testing files;  
And default command is not useful when guest use a big size virtual memory, because IO may cached in memory;
Update commands to "bonnie++ -d xxx  -n 500  -s 2048M -r 1024M" to fix  memory to 1024M  and create 2048M file to ensure IO not be cached;