vmm: config: Add poll_queue property to DiskConfig

Recently, vhost_user_block gained the ability of actively polling the
queue, a feature that can be disabled with the poll_queue property.

This change adds this property to DiskConfig, so it can be used
through the "disk" argument.

For the moment, it can only be used when vhost_user=true, but this
will change once virtio-block gets the poll_queue feature too.

Fixes: #787

Signed-off-by: Sergio Lopez <slp@redhat.com>

vmm/src/api/openapi/cloud-hypervisor.yaml

@@ -297,6 +297,9 @@ components:
         wce:
           type: boolean
           default: true
+        poll_queue:
+          type: boolean
+          default: true
 
     NetConfig:
       type: object