build: require newer netcf when it is available

When building rpms for newer Fedora or RHEL, take advantage of the
newer netcf packaging to guarantee interface snapshot support.

* libvirt.spec.in (BuildRequires): Bump minimum version on
platforms that support netcf 0.1.8.

libvirt.spec.in

@@ -424,8 +424,12 @@ BuildRequires: libcap-ng-devel >= 0.5.0
 BuildRequires: libssh2-devel
 %endif
 %if %{with_netcf}
+%if 0%{?fedora} >= 16 || 0%{?rhel} >= 6
+BuildRequires: netcf-devel >= 0.1.8
+%else
 BuildRequires: netcf-devel >= 0.1.4
 %endif
+%endif
 %if %{with_esx}
 %if 0%{?fedora} >= 9 || 0%{?rhel} >= 6
 BuildRequires: libcurl-devel