xvm-iscsi-config: print restart message only if settings actually changed
also, don't fail in prerm if /etc/invirt/conf.d/iscsi missing
svn path=/trunk/packages/xvm-iscsi-config/; revision=1925
- [DH] debian/changelog
- [DH] debian/xvm-iscsi-config.postinst
- [DH] debian/xvm-iscsi-config.prerm