vhost_task: Handle SIGKILL by flushing work and exiting

In the Linux kernel, the following vulnerability has been resolved:

vhost_task: Handle SIGKILL by flushing work and exiting

Instead of lingering until the device is closed, this has us handle

SIGKILL by:

1. marking the worker as killed so we no longer try to use it with

new virtqueues and new flush operations.

2. setting the virtqueue to worker mapping so no new works are queued.

3. running all the exiting works.