tests/qtest/vhost-user-blk-test: use memory-backend-shm

tests/qtest/vhost-user-blk-test: use memory-backend-shm

`memory-backend-memfd` is available only on Linux while the new
`memory-backend-shm` can be used on any POSIX-compliant operating
system. Let's use it so we can run the test in multiple environments.

Since we are here, let`s remove `share=on` which is the default for shm
(and also for memfd).

Acked-by: Thomas Huth <thuth@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20240618100527.145883-1-sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>