numa: avoid failure in nodememstats on non-NUMA systems

numa: avoid failure in nodememstats on non-NUMA systems

libvirt reports a fake NUMA topology in virConnectGetCapabilities
even if built without numactl support. The fake NUMA topology consists
of a single cell representing the host's cpu and memory resources.
Currently this is the case for ARM and s390[x] RPM builds.

A client iterating over NUMA cells obtained via virConnectGetCapabilities
and invoking virNodeGetMemoryStats on them will see an internal failure
"NUMA isn't available on this host" from virNumaGetMaxNode. An example
for such a client is VDSM.

Since the intention seems to be that libvirt always reports at least
a single cell it is necessary to return "fake" node memory statistics
matching the previously reported fake cell in case NUMA isn't supported
on the system.

Signed-off-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>