libxl: Fix domxml-to-native conversion

libxl: Fix domxml-to-native conversion

Similar to commit 57d084febe, another case of the libxl driver not
adapting to modular daemons. When converting configuration that
contains a type='network' interface, the converter calls
virNetworkLookupByName, passing the hypervisor connection object
instead of a connection to virtnetworkd. E.g.

> cat dom.xml
...
    <interface type='network'>
      <source network='default'/>
    </interface>
...
> virsh net-info default
Name:           default
UUID:           25a5b089-1e71-4956-99aa-df2213bbb407
Active:         yes
Persistent:     no
Autostart:      no
Bridge:         virbr0
> virsh domxml-to-native xen-xl dom.xml
error: Network not found: default

Acquire a connection to virtnetworkd and use it when calling
virNetwork* APIs.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>