block: Use warn_report() & friends to report warnings

Calling error_report() in a function that takes an Error ** argument
is suspicious.  Convert a few that are actually warnings to
warn_report().

While there, split warnings consisting of multiple sentences to
conform to conventions spelled out in warn_report()'s contract, and
improve a rather useless warning in sheepdog.c.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Lieven <pl@kamp.de>
Cc: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20181017082702.5581-4-armbru@redhat.com>

Drop changes to "without an explicit read-only=on" warnings, because
there's a series removing them pending.  Also drop a cc: to a former
Sheepdog maintainer.

Reviewed-by: Kevin Wolf <kwolf@redhat.com>

block/iscsi.c

@@ -1844,7 +1844,7 @@ static int iscsi_open(BlockDriverState *bs, QDict *options, int flags,
     iscsi_set_timeout(iscsi, timeout);
 #else
     if (timeout) {
-        error_report("iSCSI: ignoring timeout value for libiscsi <1.15.0");
+        warn_report("iSCSI: ignoring timeout value for libiscsi <1.15.0");
     }
 #endif
 

block/rbd.c

@@ -750,8 +750,8 @@ static int qemu_rbd_open(BlockDriverState *bs, QDict *options, int flags,
         /* Take care whenever deciding to actually deprecate; once this ability
          * is removed, we will not be able to open any images with legacy-styled
          * backing image strings. */
-        error_report("RBD options encoded in the filename as keyvalue pairs "
-                     "is deprecated");
+        warn_report("RBD options encoded in the filename as keyvalue pairs "
+                    "is deprecated");
     }
 
     /* Remove the processed options from the QDict (the visitor processes

block/sheepdog.c

@@ -572,7 +572,7 @@ static int connect_to_sdog(BDRVSheepdogState *s, Error **errp)
     if (s->addr->type == SOCKET_ADDRESS_TYPE_INET && fd >= 0) {
         int ret = socket_set_nodelay(fd);
         if (ret < 0) {
-            error_report("%s", strerror(errno));
+            warn_report("can't set TCP_NODELAY: %s", strerror(errno));
         }
     }