Update git submodules

* Update docs/submodules/vswitchperf from branch 'master'
  - Merge "trex: Improve error handling"
  - trex: Improve error handling

    Print an error message in case, that vsperf is not able
    to connect to the T-Rex server. User is instructed
    to check if T-Rex server is up and running.

    Change-Id: I70c6d27fa8331921d43e25c5eaa3638ded6c015b
    Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
    Reviewed-by: Jose Lausuch <jalausuch@suse.com>
    Reviewed-by: Al Morton <acmorton@att.com>
    Reviewed-by: Christian Trautman <ctrautma@redhat.com>
    Reviewed-by: Sridhar Rao <sridhar.rao@spirent.com>
    Reviewed-by: Trevor Cooper <trevor.cooper@intel.com>

diff --git a/docs/submodules/vswitchperf b/docs/submodules/vswitchperf
index 3e4a1be..78de0c6 160000 (submodule)
--- a/docs/submodules/vswitchperf
+++ b/docs/submodules/vswitchperf
@@ -1 +1 @@
-Subproject commit 3e4a1be91d487d4a73fc80f1c3f23ebf78c15953
+Subproject commit 78de0c6b53db31ae0d77aaa79de015a73e981895