[cros_vm]: Use separate directories for different ports.

This allows us to launch multiple VMs with different ports.

BUG=chromium:782664
TEST=Launch multiple VMs with different ports.

Change-Id: I5a443d9d4e406a49d57e9ea8788fe42b7c4021e0
Reviewed-on: https://chromium-review.googlesource.com/871140
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cros_vm.py b/scripts/cros_vm.py
index 8139953..89f6976 100644
--- a/scripts/cros_vm.py
+++ b/scripts/cros_vm.py
@@ -62,7 +62,8 @@
     self.stop = opts.stop
     self.cmd = opts.args[1:] if opts.cmd else None
 
-    self.vm_dir = os.path.join(osutils.GetGlobalTempDir(), 'cros_vm')
+    self.vm_dir = os.path.join(osutils.GetGlobalTempDir(),
+                               'cros_vm_%d' % self.ssh_port)
     if os.path.exists(self.vm_dir):
       # For security, ensure that vm_dir is not a symlink, and is owned by us or
       # by root.