virDomainNumatuneGetMode: Report if numatune was defined

virDomainNumatuneGetMode: Report if numatune was defined

So far, we are not reporting if numatune was even defined. The
value of zero is blindly returned (which maps onto
VIR_DOMAIN_NUMATUNE_MEM_STRICT). Unfortunately, we are making
decisions based on this value. Instead, we should not only return
the correct value, but report to the caller if the value is valid
at all.

For better viewing of this patch use '-w'.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>