migration: flush migration data to disk.

This patch increases robustness when migrating to a file with
two little changes:

 (1) Before closing the migration file handle checks if it happens to be
     a regular file and if so it issues a fsync.  This way the data is
     flushed to disk before qemu sends the migration completed event.
 (2) It adds error checking.  In case either fsync or close syscall
     fails pass up the error (and fail migration).

[ v2: return -errno instead of -1 ]

Cc: Juan Quintela <quintela@redhat.com>
Cc: Jiri Denemark <jdenemar@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

migration-fd.c

@@ -42,10 +42,31 @@ static int fd_write(MigrationState *s, const void * buf, size_t size)
 
 static int fd_close(MigrationState *s)
 {
+    struct stat st;
+    int ret;
+
     DPRINTF("fd_close\n");
     if (s->fd != -1) {
-        close(s->fd);
+        ret = fstat(s->fd, &st);
+        if (ret == 0 && S_ISREG(st.st_mode)) {
+            /*
+             * If the file handle is a regular file make sure the
+             * data is flushed to disk before signaling success.
+             */
+            ret = fsync(s->fd);
+            if (ret != 0) {
+                ret = -errno;
+                perror("migration-fd: fsync");
+                return ret;
+            }
+        }
+        ret = close(s->fd);
         s->fd = -1;
+        if (ret != 0) {
+            ret = -errno;
+            perror("migration-fd: close");
+            return ret;
+        }
     }
     return 0;
 }