[libvirt] Add support for image write mode in virtualbox driver

Hi everyone, We currently using libvirt for PYTI GSOC Project (it works very well!) and we encounter a problem. We use VM with two disk images, one which must to be snapshoted/rollbacked and the other one which must be not. We use VirtualBox as hypervisor and to do so, VirtualBox define "Special image write modes" (http://www.virtualbox.org/manual/ch05.html#hdimagewrites). What we need is to set the type of one image to "writethrough". This information seems defined in VM config file. For instance: <HardDisk uuid="{402aef07-4df8-4518-831c-eeb9ae2a32d2}" location="/home/boris/VirtualBox VMs/PYTI.vdi" format="VDI" type="Normal"/> <HardDisk uuid="{63acf6d9-b7db-4d8d-8389-1276c3d500bb}" location="/home/boris/VirtualBox VMs/adhock.vdi" format="VDI" type="Writethrough"/> I found two pages mentioning the type of medium, this one: http://www.virtualbox.org/sdkref/_virtual_box_8idl.html#b4abb9d1b9e44b997... and this one: http://www.virtualbox.org/sdkref/interface_i_medium_attachment.html. Hope it will help. Cheers, FELD Boris