api: Fix object.VirtualDiskManager::Copy

This patch fixes object.VirtualDiskManager::CopyVirtualDisk
so it takes an interface instead of a concrete type for the
destination disk spec.

Signed-off-by: akutz <akutz@vmware.com>

Author: akutz

object/virtual_disk_manager.go

@@ -41,7 +41,7 @@ func (m VirtualDiskManager) CopyVirtualDisk(
 	ctx context.Context,
 	sourceName string, sourceDatacenter *Datacenter,
 	destName string, destDatacenter *Datacenter,
-	destSpec *types.VirtualDiskSpec, force bool) (*Task, error) {
+	destSpec types.BaseVirtualDiskSpec, force bool) (*Task, error) {
 
 	req := types.CopyVirtualDisk_Task{
 		This:       m.Reference(),