qemu_command: do not use host-nodes for system memory

qemu_command: do not use host-nodes for system memory

Commit 88957116c9d3cb4705380c3702c9d4315fb500bb switched to use
memory-backend-* for regular VM memory as well. That change indirectly
started using 'host-nodes' for system memory which results in QEMU
calling mbind() to bind the system memory to specific NUMA node if the
VM XML contains the configuration similar to this:

  ...
  <numatune>
    <memory mode='strict' nodeset='0'/>
  </numatune>
  ...

Once the VM was started with that configuration it was no longer
possible to change the memory NUMA nodeset.

Fixes: 677c90cc1d1fcb3aba09b5d4f0f8f83099911775
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>