vdpa: reorder listener assignment

vdpa: reorder listener assignment

Since commit f6fe3e333f ("vdpa: move memory listener to
vhost_vdpa_shared") this piece of code repeatedly assign
shared->listener members.  This was not a problem as it was not used
until device start.

However next patches move the listener registration to this
vhost_vdpa_init function.  When the listener is registered it is added
to an embedded linked list, so setting its members again will cause
memory corruption to the linked list node.

Do the right thing and only set it in the first vdpa device.

Tested-by: Lei Yang <leiyang@redhat.com>
Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Jonah Palmer <jonah.palmer@oracle.com>
Message-Id: <20250522145839.59974-6-jonah.palmer@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>