vswitchd: Require "target" column to be unique in OVS database.

Commit cc7ecee (vswitchd: Add unique indexes for some columns.) says,
in part:

    With this commit, the database server itself rejects attempts to add
    Port or Interface records with duplicate names or Controller or
    Manager records with duplicate targets.

but in fact didn't change the Controller table as described.  This commit
fixes that.

This commit updates the schema version number's major version, because this
is a potentially non-backward compatible change, if some user depended on
the ability to add Controller records with duplicate targets.  However, if
anyone thinks this is a bad idea, then I'm open to discussion.

Reported-by: Natasha Gude <[email protected]>
Signed-off-by: Ben Pfaff <[email protected]>
Acked-by: Ethan Jackson <[email protected]>

vswitchd/vswitch.ovsschema

@@ -1,6 +1,6 @@
 {"name": "Open_vSwitch",
  "version": "7.0.0",
- "cksum": "3537583872 17299",
+ "cksum": "438518409 17329",
  "tables": {
    "Open_vSwitch": {
      "columns": {
@@ -428,7 +428,8 @@
          "ephemeral": true},
        "status": {
          "type": {"key": "string", "value": "string", "min": 0, "max": "unlimited"},
-         "ephemeral": true}}},
+         "ephemeral": true}},
+     "indexes": [["target"]]},
    "Manager": {
      "columns": {
        "target": {