qemu-iotests: add testcase for bz #1857490

Test that we can't write-share raw luks images by default,
but we still can with share-rw=on

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20200719122059.59843-3-mlevitsk@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>