As per my understanding, All Rest APIs related to VM are not available in current vSphere releases, we might expect them in the next vSphere major releases.
For now to get this VM committed storage use vSphere Management SDK (SOAP based):
VM committed storage(VirtualMachineStorageSummary ) - Total storage space, in bytes, committed to this virtual machine across all data stores. Essentially an aggregate of the property committed across all data stores that this virtual machine is located on.
Here is the sample code to get the VM committed storage using VIJava API:
ServiceInstance si = new ServiceInstance(new URL("https://" + vcIPaddress + "/sdk"), userName, password, true);
VirtualMachine vm = (VirtualMachine) new InventoryNavigator(si.getRootFolder())
.searchManagedEntity("VirtualMachine", "<VM_Name>");
System.out.println("VM Committed storage::"+vm.getSummary().getStorage().getCommitted());
System.out.println("VM UnCommitted storage::"+vm.getSummary().getStorage().getUncommitted());