rdma: introduce capability x-rdma-pin-all

rdma: introduce capability x-rdma-pin-all

This capability allows you to disable dynamic chunk registration
for better throughput on high-performance links.

For example, using an 8GB RAM virtual machine with all 8GB of memory in
active use and the VM itself is completely idle using a 40 gbps infiniband link:

1. x-rdma-pin-all disabled total time: approximately 7.5 seconds @ 9.5 Gbps
2. x-rdma-pin-all enabled total time: approximately 4 seconds @ 26 Gbps

These numbers would of course scale up to whatever size virtual machine
you have to migrate using RDMA.

Enabling this feature does *not* have any measurable affect on
migration *downtime*. This is because, without this feature, all of the
memory will have already been registered already in advance during
the bulk round and does not need to be re-registered during the successive
iteration rounds.

Reviewed-by: Juan Quintela <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Reviewed-by: Chegu Vinod <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Tested-by: Chegu Vinod <[email protected]>
Tested-by: Michael R. Hines <[email protected]>
Signed-off-by: Michael R. Hines <[email protected]>
Signed-off-by: Juan Quintela <[email protected]>