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.

nixos/lib/test-driver/test-driver.py

@@ -615,7 +615,6 @@ class Machine:
             # Copy the file to the shared directory inside VM
             self.succeed(make_command(["mkdir", "-p", vm_shared_temp]))
             self.succeed(make_command(["cp", "-r", vm_src, vm_intermediate]))
-            self.succeed("sync")
             abs_target = out_dir / target_dir / vm_src.name
             abs_target.parent.mkdir(exist_ok=True, parents=True)
             # Copy the file from the shared directory outside VM

nixos/modules/virtualisation/qemu-vm.nix

@@ -567,7 +567,7 @@ in
         "/tmp/xchg" =
           { device = "xchg";
             fsType = "9p";
-            options = [ "trans=virtio" "version=9p2000.L" "cache=loose" ];
+            options = [ "trans=virtio" "version=9p2000.L" ];
             neededForBoot = true;
           };
         "/tmp/shared" =

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

pkgs/build-support/vm/windows/controller/default.nix

@@ -53,7 +53,7 @@ let
       store /fs/nix/store
 
     mount -t 9p \
-      -o trans=virtio,version=9p2000.L,cache=loose \
+      -o trans=virtio,version=9p2000.L \
       xchg /fs/xchg
 
     echo root:x:0:0::/root:/bin/false > /fs/etc/passwd