mirror: use bdrv_drained_begin/bdrv_drained_end

Ensure that there are no changes between the last check to
bdrv_get_dirty_count and the switch to the target.

There is already a bdrv_drained_end call, we only need to ensure
that bdrv_drained_begin is not called twice.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-Id: <1477565348-5458-4-git-send-email-pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
