qemu: Use thread-context even with numatune's restrictive mode

qemu: Use thread-context even with numatune's restrictive mode

We cannot use host-nodes attribute for it, but there is no reason for us
to skip the preallocation optimisation using thread-context in such
case.  Thankfully returning the proper nodemask from
qemuBuildMemoryBackendProps is enough to trigger this.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>