zones: fix jpictl dump by explicitly omitting Machine.logger and Zone.logger

if they were fields, as in Zones, they would be ignored automatically.
but they aren't

Signed-off-by: Alejandro Mery <amery@jpi.io>

pkg/zones/zones.go

@@ -87,8 +87,8 @@ func FilterMachines(m MachineIterator, cond func(*Machine) bool) (Machines, int)
 
 // Zone represents one zone in a cluster
 type Zone struct {
-	zones *Zones
-	logger
+	zones  *Zones
+	logger `toml:"-" json:"-" yaml:"-"`
 
 	ID   int    `toml:"id"`
 	Name string `toml:"name"`