Don't require iptables-ipv6 on >= F17 and RHEL >= 7

We only need ip6tables, which is now a part of the iptables package:
http://pkgs.fedoraproject.org/cgit/iptables.git/commit/?h=1392835

This needlessly pulls in iptables-services:
https://bugzilla.redhat.com/show_bug.cgi?id=1101510
which can conflict with firewalld:
https://bugzilla.redhat.com/show_bug.cgi?id=1101484

libvirt.spec.in

@@ -522,7 +522,9 @@ BuildRequires: libapparmor-devel
 %if %{with_network}
 BuildRequires: dnsmasq >= 2.41
 BuildRequires: iptables
+    %if (0%{?fedora} && 0%{?fedora} < 17) || (0%{?rhel} && 0{?rhel} < 7)
 BuildRequires: iptables-ipv6
+    %endif
 BuildRequires: radvd
 %endif
 %if %{with_nwfilter}
@@ -766,7 +768,9 @@ Requires: libvirt-daemon = %{version}-%{release}
 Requires: dnsmasq >= 2.41
 Requires: radvd
 Requires: iptables
+    %if (0%{?fedora} && 0%{?fedora} < 17) || (0%{?rhel} && 0{?rhel} < 7)
 Requires: iptables-ipv6
+    %endif
 
 %description daemon-driver-network
 The network driver plugin for the libvirtd daemon, providing
@@ -781,7 +785,9 @@ Summary: Nwfilter driver plugin for the libvirtd daemon
 Group: Development/Libraries
 Requires: libvirt-daemon = %{version}-%{release}
 Requires: iptables
+    %if (0%{?fedora} && 0%{?fedora} < 17) || (0%{?rhel} && 0{?rhel} < 7)
 Requires: iptables-ipv6
+    %endif
 Requires: ebtables
 
 %description daemon-driver-nwfilter