qemu: Memory pre-pinning support for RDMA migration

qemu: Memory pre-pinning support for RDMA migration

RDMA Live migration requires registering memory with the hardware, and
thus QEMU offers a new 'capability' to pre-register / mlock() the guest
memory in advance for higher RDMA performance before the migration
begins. This capability is disabled by default, which means QEMU will
register the memory with the hardware in an on-demand basis.

This patch exposes this capability with the following example usage:

virsh migrate --live --rdma-pin-all --migrateuri rdma://hostname domain qemu+ssh://hostname/system

Signed-off-by: Michael R. Hines <mrhines@us.ibm.com>
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>