Fix incompatible change from 77982e

77982e7#diff-2f03be069459ebc91fcf0607d4d686d8R832
checks if the source network is a bridge but doesn't set the bridge
value correctly in the Terraform state just after.

See https://godoc.org/github.com/libvirt/libvirt-go-xml#DomainInterfaceSourceNetworkhttps://godoc.org/github.com/libvirt/libvirt-go-xml#DomainInterfaceSourceBridge
for reference.

Also, in that case, .Source.Network is not defined so better let it to the
default value.

libvirt/resource_libvirt_domain.go

@@ -840,7 +840,7 @@ func resourceLibvirtDomainRead(d *schema.ResourceData, meta interface{}) error {
 				}
 			}
 		} else if networkInterfaceDef.Source.Bridge != nil {
-			netIface["bridge"] = networkInterfaceDef.Source.Bridge
+			netIface["bridge"] = networkInterfaceDef.Source.Bridge.Bridge
 		} else if networkInterfaceDef.Source.Direct != nil {
 			switch networkInterfaceDef.Source.Direct.Mode {
 			case "vepa":