libxl: Don't use dom->conn to lookup virNetwork

libxl: Don't use dom->conn to lookup virNetwork

When using the monolithic daemon, then dom->conn has all driver
tables filled in properly and thus it's safe to call an API other
than virDomain*(). However, when using split daemons then
dom->conn has only hypervisor driver table set
(dom->conn->driver) and the rest is NULL. Therefore, if we want
to call a non-domain API (virNetworkLookupByName() in this case),
we have obtain the cached connection object accessible via
virGetConnectNetwork().

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Cole Robinson <crobinso@redhat.com>