This helper has solely to do with virObjects. Move it together with
other virObject stuff.

This also avoids the potential problem where VIR_AUTOUNREF uses
virObjectAutoUnref which is defined in virobject.h.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>