Remove duplicate of 'Dynamo: Amazon's Highly Available Key-value store'

Those two PDF files have the same content, except their layout is merely
different.
Add COPS and Eiger paper

These two papers talks about how to support causal consistency on
geo-distributed scale. They appeared in SOSP'11 and NSDI'13
Add Xen and VM live migration paper

The Xen paper talked about the designing of virtual machine
hypervisor, which lead to the implementation of Xen - the
virtualization component of Amazon EC2.

Live migration paper discusses how to transparently migrate
virtual machines across hosts without down time.
Merge pull request #39 from santazhang/master

Remove dups, add papers on distributed systems and virtualization