libxl: fix node ranges in libxlNodeGetCellsFreeMemory()

libxl: fix node ranges in libxlNodeGetCellsFreeMemory()

introduced by cs 4b9eec50fe2c23343 ("libxl: implement per
NUMA node free memory reporting"). What was wrong was that
libxl_get_numainfo() put in nr_nodes the actual number of
host NUMA nodes, not the highest node ID (like libnuma's
numa_max_node() does instead).

While at it, turn the failure of libxl_get_numainfo() from
a simple warning to a proper error, as requested during the
review of another patch of the original series.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Cc: Daniel P. Berrange <berrange@redhat.com>