vdpa_sim: move buffer allocation in the devices

Currently, the vdpa_sim core does not use the buffer, but only
allocates it.

The buffer is used by devices differently, and some future devices
may not use it. So let's move all its management inside the devices.

Add a new `free` device callback called to clean up the resources
allocated by the device.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20230407133658.66339-2-sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>