Now builds with ocaml-4.00.1 again

• Several bugfixes
• Reinstate WLB code
• Fix build on ocaml 4.00.1

Now with vhd import support

We have

• vhd import
• migration bugfix
• lots of other bug fixes

Approach vector to 2.0

Merge pull request #1769 from djs55/smapiv2

Remove the hardcoded disabling of ballooning for dom0