fix UnmountDevice failure on Windows

[andyzhangx]

What this PR does / why we need it:
fix UnmountDevice failure on Windows, root cause is GetMountRefs on windows is not correctly implemented.
Error code is like following:

E0912 04:11:58.069277    4308 nestedpendingoperations.go:267] Operation for "\"kubernetes.io/azure-disk//subscriptions/xxx/resourceGroups/andy-k8swin1120beta1/providers/Microsoft.Compute/disks/andy-k8swin1120beta1-dynamic-pvc-ec35a4b1-b637-11e8-bb79-000d3af7dab4\"" failed. No retries permitted until 2018-09-12 04:14:00.0692771 +0000 GMT m=+572.648837101 (durationBeforeRetry 2m2s). Error: "GetDeviceMountRefs check failed for volume \"pvc-ec35a4b1-b637-11e8-bb79-000d3af7dab4\" (UniqueName: \"kubernetes.io/azure-disk//subscriptions/xxx/resourceGroups/andy-k8swin1120beta1/providers/Microsoft.Compute/disks/andy-k8swin1120beta1-dynamic-pvc-ec35a4b1-b637-11e8-bb79-000d3af7dab4\") on node \"18986k8s9001\" : The device mount path \"\\\\var\\\\lib\\\\kubelet\\\\plugins\\\\kubernetes.io\\\\azure-disk\\\\mounts\\\\m1433781257\" is still mounted by other references [c:\\var\\lib\\kubelet\\plugins\\kubernetes.io\\azure-disk\\mounts\\m1433781257 F:\\]"

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #68512

Special notes for your reviewer:
On Windows, there is no place to query all mount points like /proc/mounts on Linux, so I actually did an empty implementation for GetMountRefs on Windows, which could skip the below error check:

kubernetes/pkg/volume/util/operationexecutor/operation_generator.go

Lines 756 to 758 in 426ef9d

	if err != nil || mount.HasMountRefs(deviceMountPath, refs) {
	if err == nil {
	err = fmt.Errorf("The device mount path %q is still mounted by other references %v", deviceMountPath, refs)

Release note:

fix UnmountDevice failure on Windows

/kind bug
/sig windows
@PatrickLang

[andyzhangx]

/test pull-kubernetes-cross

[andyzhangx]

/assign @jsafrane @msau42

[msau42]

/lgtm

[andyzhangx]

@jsafrane PTAL

[andyzhangx]

/test pull-kubernetes-kubemark-e2e-gce-big

[jsafrane]

/approve

[andyzhangx]

/test pull-kubernetes-cross

[andyzhangx]

rebased, and I changed the test code a little @jsafrane PTAL

[jsafrane]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andyzhangx, jsafrane, msau42

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• pkg/util/mount/OWNERS [jsafrane]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment