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_call.err b/tests/msc_vlr/msc_vlr_test_call.err
index c7c7d8d..d17d44b 100644
--- a/tests/msc_vlr/msc_vlr_test_call.err
+++ b/tests/msc_vlr/msc_vlr_test_call.err
@@ -91,6 +91,7 @@
 DVLR GSUP rx 17: 10010809710000000156f00804032443f2
 DREF VLR subscr IMSI:901700000010650 usage increases to: 2
 DVLR IMSI:901700000010650 has MSISDN:42342
+DVLR SUBSCR(MSISDN:42342) VLR: update for IMSI=901700000010650 (MSISDN=42342, used=2)
 DVLR GSUP tx: 12010809710000000156f0
 GSUP --> HLR: OSMO_GSUP_MSGT_INSERT_DATA_RESULT: 12010809710000000156f0
 DREF VLR subscr MSISDN:42342 usage decreases to: 1
@@ -439,6 +440,7 @@
 DVLR GSUP rx 17: 10010809710000000156f00804032443f2
 DREF VLR subscr IMSI:901700000010650 usage increases to: 2
 DVLR IMSI:901700000010650 has MSISDN:42342
+DVLR SUBSCR(MSISDN:42342) VLR: update for IMSI=901700000010650 (MSISDN=42342, used=2)
 DVLR GSUP tx: 12010809710000000156f0
 GSUP --> HLR: OSMO_GSUP_MSGT_INSERT_DATA_RESULT: 12010809710000000156f0
 DREF VLR subscr MSISDN:42342 usage decreases to: 1