vdpa-dev: Fix initialisation order to restore VDUSE compatibility

vdpa-dev: Fix initialisation order to restore VDUSE compatibility

VDUSE requires that virtqueues are first enabled before the DRIVER_OK
status flag is set; with the current API of the kernel module, it is
impossible to enable the opposite order in our block export code because
userspace is not notified when a virtqueue is enabled.

This requirement also mathces the normal initialisation order as done by
the generic vhost code in QEMU. However, commit 6c482547 accidentally
changed the order for vdpa-dev and broke access to VDUSE devices with
this.

This changes vdpa-dev to use the normal order again and use the standard
vhost callback .vhost_set_vring_enable for this. VDUSE devices can be
used with vdpa-dev again after this fix.

vhost_net intentionally avoided enabling the vrings for vdpa and does
this manually later while it does enable them for other vhost backends.
Reflect this in the vhost_net code and return early for vdpa, so that
the behaviour doesn't change for this device.

Cc: qemu-stable@nongnu.org
Fixes: 6c4825476a43 ('vdpa: move vhost_vdpa_set_vring_ready to the caller')
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20240315155949.86066-1-kwolf@redhat.com>
Reviewed-by: Eugenio Pérez <eperezma@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 2c66de61f88dc9620a32239f7dd61524a57f66b0)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>