vm's should be able preallocate .img file

Pre-allocating the .img could improve the performance of our vm testing a bit. From jibel:

<jibel> doanac, you could use -o preallocation=metadata during disk creation
<jibel> doanac, then fallocate on this file to preallocate blocks on disk
<jibel> you'll end with a file that will be the size of the final disk of the VM and will improve a bit performances during testing

Let's do a quick hack first and measure the performance of this. Then decide if we should support it and whether or not it should be enabled by default (ie - because we'll potentially start using a lot more disk space).