vdpa: use generic driver_override infrastructure

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

vdpa: use generic driver_override infrastructure

When a driver is probed through __driver_attach(), the bus' match() callback is called without the device lock held, thus accessing the driver_override field without a lock, which can cause a UAF.

Fix this by using the driver-core driver_override infrastructure taking care of proper locking internally.

Note that calling match() from __driver_attach() without the device lock held is intentional. [1]