qemu: fix live pinning to memory node on NUMA system

qemu: fix live pinning to memory node on NUMA system

Ever since the subcpusets(vcpu,emulator) were introduced, the parent
cpuset cannot be modified to remove the nodes that are in use by the
subcpusets.
The fix is to break the memory node modification into three steps:
 1. assign new nodes into the parent,
 2. change the nodes in the child nodes,
 3. remove the old nodes on the parent node.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1009880

Signed-off-by: Shivaprasad G Bhat <sbhat@linux.vnet.ibm.com>
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>