numatune: setting --mode does not work well

numatune: setting --mode does not work well

When trying to set numatune mode directly using virsh numatune command,
correct error is raised, however numatune structure was not deallocated,
thus resulting in creating an empty numatune element in the guest XML,
if none was present before. Running the same command aftewards results
in a successful change with broken XML structure. Patch fixes the
deallocation problem as well as checking for invalid attribute
combination VIR_DOMAIN_NUMATUNE_PLACEMENT_AUTO + a nonempty nodeset.

Resolves https://bugzilla.redhat.com/show_bug.cgi?id=1129998