virtio-mmio: don't break lifecycle of vm_dev

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

virtio-mmio: don't break lifecycle of vm_dev

vm_dev has a separate lifecycle because it has a 'struct device'

embedded. Thus, having a release callback for it is correct.

Allocating the vm_dev struct with devres totally breaks this protection,

though. Instead of waiting for the vm_dev release callback, the memory

is freed when the platform_device is removed. Resulting in a

use-after-free when finally the callback is to be called.

To easily see the problem, compile the kernel with

CONFIG_DEBUG_KOBJECT_RELEASE and unbind with sysfs.

The fix is easy, don't use devres in this case.

Found during my research about object lifetime problems.