client.go: set volume as usable when in migrating state

when volume is in Migrating it is fully usable to consume (like
Avaiable)

issue: LBM1-21393

pkg/lb/client.go

@@ -107,7 +107,7 @@ type Volume struct {
 }
 
 func (v *Volume) IsAccessible() bool {
-	return v.State == VolumeAvailable || v.State == VolumeUpdating
+	return v.State == VolumeAvailable || v.State == VolumeUpdating || v.State == VolumeMigrating
 }
 
 func (v *Volume) IsWritable() bool {