Pick default datastore for extra disks (#897)

Pick default datastore for extra disks

When cloning virtual machines it is possible to define additional disks
to be created. If a datastore is not defined either for the disk or for
the virtual machine then the creation of the additional virtual machines
will fail.

This change introduces a fallback where if a datastore is not set we
pick the first datastore found that is associated with the virtual
machine.

vsphere/internal/virtualdevice/virtual_machine_disk_subresource.go

@@ -3,6 +3,7 @@ package virtualdevice
 import (
 	"errors"
 	"fmt"
+	"github.com/terraform-providers/terraform-provider-vsphere/vsphere/internal/helper/virtualmachine"
 	"log"
 	"math"
 	"path"
@@ -1695,8 +1696,24 @@ func (r *DiskSubresource) assignBackingInfo(disk *types.VirtualDisk) error {
 	if dsID == "" || dsID == diskDatastoreComputedName {
 		// Default to the default datastore
 		dsID = r.rdd.Get("datastore_id").(string)
-	}
 
+		if dsID == "" {
+			vmObj, err := virtualmachine.FromUUID(r.client, r.rdd.Id())
+			if err != nil {
+				return err
+			}
+
+			vmprops, err := virtualmachine.Properties(vmObj)
+			if err != nil {
+				return err
+			}
+			if len(vmprops.Datastore) == 0 {
+				return fmt.Errorf("no datastore was set and was unable to find a default to fall back to")
+			}
+			dsID = vmprops.Datastore[0].Value
+
+		}
+	}
 	ds, err := datastore.FromID(r.client, dsID)
 	if err != nil {
 		return err