Fix for #228, #232, #632, #627, some Ui improvements (disks) and better XEN support

Updated TN model for Drive : 
 - added 3 new icons (DISK,NETWORK,FLOPPY)
 - better handling when creating and editing new disks
 - added XEN handling (very different from KVM), it fixes a lot of things

Updated TN model for Domain : 
 - No longer use KVM as default domain (XEN is here now :) )

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