Fix testsuite (or code) " Extra data in disabled network"

[dmacvicar]

=== RUN   TestAccLibvirtNetwork_DNSForwarders
    resource_libvirt_network_test.go:190: Step 1/1 error: Error running apply: exit status 1

        Error: error defining libvirt network: XML error: Extra data in disabled network 'ksltvqtgwt' -   <network>
              <name>ksltvqtgwt</name>
              <forward mode="nat"></forward>
              <bridge stp="on"></bridge>
              <domain name="k8s.local"></domain>
              <dns enable="no">
                  <forwarder addr="8.8.8.8"></forwarder>
                  <forwarder domain="my.domain.com" addr="10.10.0.67"></forwarder>
                  <forwarder domain="hello.com"></forwarder>
              </dns>
              <ip address="10.17.3.1" family="ipv4" prefix="24">
                  <dhcp>
                      <range start="10.17.3.2" end="10.17.3.254"></range>
                  </dhcp>
              </ip>
              <options xmlns="http://libvirt.org/schemas/network/dnsmasq/1.0"></options>
          </network>

          with libvirt_network.fjsaiezdlb,
          on terraform_plugin_test.tf line 4, in resource "libvirt_network" "fjsaiezdlb":
           4:                           resource "libvirt_network" "fjsaiezdlb" {