VLR: log subscriber update

* move log helpers to generic header
* log subscriber update

It's handy for troubleshooting issues with subscriber update via GSUP
from HLR.

Change-Id: I1958aeeb3ea99831c7e2c5ee9a6b59834baf4520
diff --git a/tests/msc_vlr/msc_vlr_test_rest.err b/tests/msc_vlr/msc_vlr_test_rest.err
index ce27bf1..f4e5f0e 100644
--- a/tests/msc_vlr/msc_vlr_test_rest.err
+++ b/tests/msc_vlr/msc_vlr_test_rest.err
@@ -125,6 +125,7 @@
 DVLR GSUP rx 17: 10010809710000004026f00804036470f1
 DREF VLR subscr IMSI:901700000004620 usage increases to: 2
 DVLR IMSI:901700000004620 has MSISDN:46071
+DVLR SUBSCR(MSISDN:46071) VLR: update for IMSI=901700000004620 (MSISDN=46071, used=2)
 DVLR GSUP tx: 12010809710000004026f0
 GSUP --> HLR: OSMO_GSUP_MSGT_INSERT_DATA_RESULT: 12010809710000004026f0
 DREF VLR subscr MSISDN:46071 usage decreases to: 1
@@ -241,6 +242,7 @@
 DVLR GSUP rx 17: 10010809710000004026f00804036470f1
 DREF VLR subscr MSISDN:46071 usage increases to: 3
 DVLR IMSI:901700000004620 has MSISDN:46071
+DVLR SUBSCR(MSISDN:46071) VLR: update for IMSI=901700000004620 (MSISDN=46071, used=3)
 DVLR GSUP tx: 12010809710000004026f0
 GSUP --> HLR: OSMO_GSUP_MSGT_INSERT_DATA_RESULT: 12010809710000004026f0
 DREF VLR subscr MSISDN:46071 usage decreases to: 2
@@ -405,6 +407,7 @@
 DVLR GSUP rx 17: 10010809710000004026f00804036470f1
 DREF VLR subscr IMSI:901700000004620 usage increases to: 2
 DVLR IMSI:901700000004620 has MSISDN:46071
+DVLR SUBSCR(MSISDN:46071) VLR: update for IMSI=901700000004620 (MSISDN=46071, used=2)
 DVLR GSUP tx: 12010809710000004026f0
 GSUP --> HLR: OSMO_GSUP_MSGT_INSERT_DATA_RESULT: 12010809710000004026f0
 DREF VLR subscr MSISDN:46071 usage decreases to: 1