qemu-vm: fix inconsistent caching of xchg dirs

xchg is advertised as a bidirectional exchange dir, but file content
transfer from host to VM fails due to caching:
If a file is read in the VM and then modified on the host, subsequent
re-reads in the VM can yield old, cached data.
This is caused by the use of 9p's cache=loose mode that is explicitly
meant for read-only mounts.

9p doesn't provide any suitable cache modes, so fix this by disabling
caching.

Also, remove a now unnecessary sync in the test driver.

pkgs/build-support/vm/default.nix

@@ -115,7 +115,7 @@ rec {
 
     echo "mounting host's temporary directory..."
     mkdir -p /fs/tmp/xchg
-    mount -t 9p xchg /fs/tmp/xchg -o trans=virtio,version=9p2000.L,cache=loose
+    mount -t 9p xchg /fs/tmp/xchg -o trans=virtio,version=9p2000.L
 
     mkdir -p /fs/proc
     mount -t proc none /fs/proc