Add instance.share_detach notification

This patch add a notification when a share is detached from an instance.

Manila is the OpenStack Shared Filesystems service.
These series of patches implement changes required in Nova to allow the shares
provided by Manila to be associated with and attached to instances using
virtiofs.

Implements: blueprint libvirt-virtiofs-attach-manila-shares

Change-Id: I203b71a39878c3929b22b53220f58c12c0a4c0b9

doc/notification_samples/instance-share_detach-end.json

@@ -0,0 +1,8 @@
+{
+    "event_type": "instance.share_detach.end",
+    "payload": {
+        "$ref": "common_payloads/InstanceActionSharePayload.json#"
+    },
+    "priority": "INFO",
+    "publisher_id": "nova-api:compute"
+}

doc/notification_samples/instance-share_detach-start.json

@@ -0,0 +1,8 @@
+{
+    "event_type": "instance.share_detach.start",
+    "payload": {
+        "$ref": "common_payloads/InstanceActionSharePayload.json#"
+    },
+    "priority": "INFO",
+    "publisher_id": "nova-api:compute"
+}