Add vhost_ops to vhost_dev struct and replace all relevant ioctls

Add vhost_ops to vhost_dev struct and replace all relevant ioctls

Decouple vhost from the Linux kernel by introducing vhost_ops. The
intention is to provide different backends - a 'kernel' backend based on
the ioctl interface, and an 'user' backend based on a UNIX domain socket
and shared memory interface.

Signed-off-by: Antonios Motakis <[email protected]>
Signed-off-by: Nikolay Nikolaev <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>