vringh: define the stride used for translation

Define a macro to be reused in the different parts of the code.

Useful for the next patches where we add more arrays to manage also
translations with user VA.

Suggested-by: Eugenio Perez Martin <eperezma@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20230404131326.44403-5-sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>