Sort destination RAMBlocks to be the same as the source

Use the order of incoming RAMBlocks from the source to record
an index number; that then allows us to sort the destination
local RAMBlock list to match the source.

Now that the RAMBlocks are known to be in the same order, this
simplifies the RDMA Registration step which previously tried to
match RAMBlocks based on offset (which isn't guaranteed to match).

Looking at the existing compress code, I think it was erroneously
relying on an assumption of matching ordering, which this fixes.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>