vdpa_sim_blk: support shared backend

The vdpa_sim_blk simulator uses a ramdisk as the backend. To test live
migration, we need two devices that share the backend to have the data
synchronized with each other.

Add a new module parameter to make the buffer shared between all devices.

The shared_buffer_mutex is used just to ensure that each operation is
atomic, but it is up to the user to use the devices knowing that the
underlying ramdisk is shared.

For example, when we do a migration, the VMM (e.g., QEMU) will guarantee
to write to the destination device, only after completing operations with
the source device.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20230407133658.66339-3-sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>