nbd: Include error names in trace messages

NBD errors were originally sent over the wire based on Linux errno
values; but not all the world is Linux, and not all platforms share
the same values.  Since a number isn't very easy to decipher on all
platforms, update the trace messages to include the name of NBD
errors being sent/received over the wire.  Tweak the trace messages
to be at the point where we are using the NBD error, not the
translation to the host errno values.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20171027104037.8319-2-eblake@redhat.com>

nbd/common.c

@@ -148,3 +148,26 @@ const char *nbd_cmd_lookup(uint16_t cmd)
         return "<unknown>";
     }
 }
+
+
+const char *nbd_err_lookup(int err)
+{
+    switch (err) {
+    case NBD_SUCCESS:
+        return "success";
+    case NBD_EPERM:
+        return "EPERM";
+    case NBD_EIO:
+        return "EIO";
+    case NBD_ENOMEM:
+        return "ENOMEM";
+    case NBD_EINVAL:
+        return "EINVAL";
+    case NBD_ENOSPC:
+        return "ENOSPC";
+    case NBD_ESHUTDOWN:
+        return "ESHUTDOWN";
+    default:
+        return "<unknown>";
+    }
+}