iotests: Improve iotest 194 to mirror data

Mirroring a completely sparse image to a sparse destination should be
practically instantaneous.  It isn't yet, but the test will be more
realistic if it has some non-zero to mirror as well as the holes.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20250509204341.3553601-20-eblake@redhat.com>

tests/qemu-iotests/194

@@ -34,6 +34,7 @@ with iotests.FilePath('source.img') as source_img_path, \
 
     img_size = '1G'
     iotests.qemu_img_create('-f', iotests.imgfmt, source_img_path, img_size)
+    iotests.qemu_io('-f', iotests.imgfmt, '-c', 'write 512M 1M', source_img_path)
     iotests.qemu_img_create('-f', iotests.imgfmt, dest_img_path, img_size)
 
     iotests.log('Launching VMs...')