nbd: trace long NBD operations

At the moment there are 2 sources of lengthy operations if configured:
* open connection, which could retry inside and
* reconnect of already opened connection
These operations could be quite lengthy and cumbersome to catch thus
it would be quite natural to add trace points for them.

This patch is based on the original downstream work made by Vladimir.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Hanna Reitz <hreitz@redhat.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

nbd/client-connection.c

@@ -23,6 +23,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "trace.h"
 
 #include "block/nbd.h"
 
@@ -210,6 +211,7 @@ static void *connect_thread_func(void *opaque)
             object_unref(OBJECT(conn->sioc));
             conn->sioc = NULL;
             if (conn->do_retry && !conn->detached) {
+                trace_nbd_connect_thread_sleep(timeout);
                 qemu_mutex_unlock(&conn->mutex);
 
                 sleep(timeout);

nbd/trace-events

@@ -73,3 +73,6 @@ nbd_co_receive_request_decode_type(uint64_t handle, uint16_t type, const char *n
 nbd_co_receive_request_payload_received(uint64_t handle, uint32_t len) "Payload received: handle = %" PRIu64 ", len = %" PRIu32
 nbd_co_receive_align_compliance(const char *op, uint64_t from, uint32_t len, uint32_t align) "client sent non-compliant unaligned %s request: from=0x%" PRIx64 ", len=0x%" PRIx32 ", align=0x%" PRIx32
 nbd_trip(void) "Reading request"
+
+# client-connection.c
+nbd_connect_thread_sleep(uint64_t timeout) "timeout %" PRIu64