test-driver.py: Share the shared dir between VMs

This changes the python test driver to match the behavior of the perl
test driver.  I.e. the directory mounted into /tmp/shared should be the
same for all machines.

This probably fixes many tests, but I found this while investigating
failures in nixos/tests/ceph-multi-node.nix.

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

@@ -221,7 +221,7 @@ class Machine:
             return path
 
         self.state_dir = create_dir("vm-state-{}".format(self.name))
-        self.shared_dir = create_dir("{}/xchg".format(self.state_dir))
+        self.shared_dir = create_dir("shared-xchg")
 
         self.booted = False
         self.connected = False