qemu: Fix xml dump of autogenerated websocket

When save/migrate a domain and we autogenerated a port, then if we
print the inactive domain config, write out a -1 for the socket value;
otherwise, it's possible that the subsequent start will fail if the
autogenerated websocket used conflicts with an existing running config
that also used autogenerated websockets.

Examples:

== A. Can not restore domain with autoconfigured websocket.

domain 1 and 2 have autoconfigured websocket.

1. domain 1 is started then, saved
2. domain 2 is started
3. domain 1 restoration is failed:

error: internal error: qemu unexpectedly closed the monitor: 2016-11-21T10:23:11.356687Z
qemu-kvm: -vnc 0.0.0.0:2,websocket=5700: Failed to start VNC server on `(null)':
Failed to bind socket: Address already in use

== B. Can not migrate domain with autoconfigured websocket.

domain 1 on host A, domain 2 on host B, both have autoconfigured websocket

1. domain 1 started, domain 2 started
2. domain 1 migration to host B is failed with the above error.

src/conf/domain_conf.c

@@ -22762,7 +22762,10 @@ virDomainGraphicsDefFormat(virBufferPtr buf,
             virBufferAsprintf(buf, " autoport='%s'",
                               def->data.vnc.autoport ? "yes" : "no");
 
-            if (def->data.vnc.websocket)
+            if (def->data.vnc.websocketGenerated &&
+                (flags & VIR_DOMAIN_DEF_FORMAT_INACTIVE))
+                virBufferAddLit(buf, " websocket='-1'");
+            else if (def->data.vnc.websocket)
                 virBufferAsprintf(buf, " websocket='%d'", def->data.vnc.websocket);
 
             virDomainGraphicsListenDefFormatAddr(buf, glisten, flags);