utils_test.libvirt: Scsi enhancements

1.Support to attach scsi disk to vms.
2.To attach multiple disks, I provide a list to contain targets for them: sda,b,c...,aa,ab,ac...
3.Create file for disk xml may take some time,before using it, verifying it.
Thanks for your reformation very much. 
I haven't re-read it after coding, because it works, haha:P.

But you make it much clear, I'll think about it based on your construction.
And there is a fault which we can't create device 'aaa...'.