block_device_mapping_v2.bus_type is missing from api-ref

block_device_mapping_v2.bus_type is missing from
POST /servers API reference. This patch add it.

Change-Id: I998cf6195d72c80649e9e0fd43be0cf37f8e2f48
Closes-Bug: #1713895

api-ref/source/parameters.yaml

@@ -1544,7 +1544,8 @@ block_device_mapping_v2:
             "volume_size": "25",
             "destination_type": "volume",
             "delete_on_termination": true,
-            "tag": "disk1" }]
+            "tag": "disk1",
+            "disk_bus": "scsi"}]
 
     In microversion 2.32, ``tag`` is an optional string attribute that can
     be used to assign a tag to the block device. This tag is then exposed to
@@ -1985,6 +1986,15 @@ disk_available_least:
   in: body
   required: true
   type: integer
+disk_bus:
+  description: |
+    Disk bus type, some hypervisors (currently only libvirt) support
+    specify this parameter. Some example disk_bus values can be: `ide`,
+    `usb`, `virtio`, `scsi`. This is not an exhaustive list as it depends
+    on the virtualization driver, and may change as more support is added.
+  in: body
+  required: false
+  type: string
 disk_config:
   description: |
     Disk configuration. The value is either:

api-ref/source/servers.inc

@@ -328,6 +328,7 @@ Request
   - block_device_mapping_v2.boot_index: boot_index
   - block_device_mapping_v2.uuid: block_device_uuid
   - block_device_mapping_v2.tag: device_tag_bdm
+  - block_device_mapping_v2.disk_bus: disk_bus
   - config_drive: config_drive
   - key_name: key_name
   - os:scheduler_hints: os:scheduler_hints