nixos/qemu-vm: virtiofsd requires shared memory

We recently switched to virtiofs for qemu-vms run on Linux
(https://github.com/NixOS/nixpkgs/pull/552774). I suspect we
only tested it with nixos tests, which just [happen to enable shared memory](673028b7b1/nixos/lib/testing/driver.nix (L259))
for the ssh backdoor. However, for folks using qemu-vm.nix outside of nixos
tests, they'll experience virtiofsd crashing with [some very obtuse
messages](https://gitlab.com/virtio-fs/virtiofsd/-/work_items/127).

Reported by @fricklerhandwerk in
<https://github.com/NixOS/nixpkgs/pull/552774#issuecomment-5668027063>.

nixos/modules/virtualisation/qemu-vm.nix

@@ -741,7 +741,11 @@ in
     };
 
     virtualisation.qemu = {
-      enableSharedMemory = mkEnableOption "shared memory";
+      enableSharedMemory = mkOption {
+        type = types.bool;
+        default = useVirtiofs; # Need shared memory for virtiofs: <https://www.qemu.org/docs/master/system/devices/virtio/vhost-user.html#shared-memory-object>
+        description = "Enable shared memory";
+      };
 
       package = mkOption {
         type = types.package;