Reset the dirty bitmap before reading to make sure we don't miss
any new data.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Lieven <pl@kamp.de>
Message-Id: <1520507908-16743-3-git-send-email-pl@kamp.de>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>