Add Task class to xapi client
VDI upload, floppy and tools support

This commit add new VDI steps named step_{attach,detach,find,upload}_vdi
  - attach/detach are self-explanatory; can specify CD or Floppy
  - find finds a VDI by name-label and stores the uuid
  - upload uploads a VDI from a local disk image and stores the uuid

In order to demonstrate the new VDI code, this commit trivially adds:
  - support for uploading ISOs again (dependent on unmerged XAPI patch)
  - initial floppy upload/attach support (dependent on currently-broken
    XAPI patch)
  - initial XenServer Tools support
Rename root_disk_size to disk_size

CreateVdi now takes an integer size
Note that disk_size is now in MB; root_disk_size was in bytes
Fixed example
Rename instance_name to vm_name

Fixed example
Rename instance_memory to vm_memory

Note that vm_memory is now in MB; instance_memory was in bytes
SetStaticMemoryRange now takes int64
Fixed example