virInsertElementsN: Rename 'add' argument

virInsertElementsN: Rename 'add' argument

The idea of @add was that the insersion/appension macros would allow
adding more than one element but this feature was never implemented.

'add' is nowadays used as a dummy variable consuming the result of the
VIR_TYPEMATCH compile time check.

Make it obvious that we don't use 'add' by renaming it to
'typematchDummy', marking it as unused and replacing all occurences
where the value was used by literal '1'.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>