libxl_migration: Resolve Coverity RESOURCE_LEAK

In libxlDomainMigrationPrepare() if the uri_in is false, then
'hostname' is allocated and used "generically" in the routine,
but not freed.  Conversely, if uri_in is true, then a uri is
allocated and hostname is set to the uri->hostname value and
likewise generically used.

At function exit, hostname wasn't free'd in the !uri_in path,
so that was added.  To just make it clearer on usage the else
path became the call to virURIFree() although I suppose technically
it didn't have to since it would be a call using (NULL)

src/libxl/libxl_migration.c

@@ -412,7 +412,10 @@ libxlDomainMigrationPrepare(virConnectPtr dconn,
     }
 
  done:
-    virURIFree(uri);
+    if (!uri_in)
+        VIR_FREE(hostname);
+    else
+        virURIFree(uri);
     if (vm)
         virObjectUnlock(vm);
     return ret;