Use automatic memory freeing for 'tmpvars' and move the allocation of
tmpvars earlier so that we are guaranteed that 'obj' will always be
appended to 'inst->filters' and thus don't need cleanup for it.
By moving the reset of 'inst' to the block when virNWFilterDefToInst
fails we can get rid of the rest of the cleanup section and remove the
'ret' variable.
Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Michal Privoznik <mprivozn@redhat.com>