migration/block-dirty-bitmap: fix dirty_bitmap_load

migration/block-dirty-bitmap: fix dirty_bitmap_load

dirty_bitmap_load_header return code is obtained but not handled. Fix
this.

Bug was introduced in b35ebdf076d697bc
"migration: add postcopy migration of dirty bitmaps" with the whole
function.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180530112424.204835-1-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
(cherry picked from commit a36f6ff46f115672cf86d0e1e7cdb1c2fa4d304b)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>