qemu_file: Use a stat64 for qemu_file_transferred

This way we can read it from any thread.
I checked that it gives the same value as the current one.  We never
use two qemu_files at the same time.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231025091117.6342-3-quintela@redhat.com>

migration/migration-stats.h

@@ -81,6 +81,10 @@ typedef struct {
      * Number of bytes sent during precopy stage.
      */
     Stat64 precopy_bytes;
+    /*
+     * Number of bytes transferred with QEMUFile.
+     */
+    Stat64 qemu_file_transferred;
     /*
      * Amount of transferred data at the start of current cycle.
      */