CP-8378: Handle legacy migration streams and suspend images

Support for handling 32bit -> 64bit migration. This has added another parameter to the Xenops migrate_receive_memory call to specify the incoming memory image format. The suspend VDI has now been changed to not have an EXT2 filesystem on it and the suspend image format when resuming from older suspend image is inferred by the presence or otherwise of this filesystem.

I have dev tested a storage migrate from Clearwater (exercising the migrate_receive_memory HTTP handler) and resuming from a suspend image created on Clearwater.

Because of the interface change this will not build correctly until https://github.com/xapi-project/xcp-idl/pull/26 is merged to xcp-idl.