parallels: do not trust the bitmaps of an image which was not closed

The inuse magic in the header says that the image was not closed
correctly. The bitmaps stored in the Format Extension are then stale by
definition: they were written by the last inactivation, and every write
which happened after it is missing from them. Nothing said so, the
bitmaps were loaded and handed out as valid, and an incremental backup
taken from one of them would silently miss the data written after the
last clean close.

Mark them inconsistent, as qcow2 does for a bitmap whose in-use flag
survived a crash. Using such a bitmap fails with an error naming it, so
the loss is reported to whoever tries to rely on it instead of being
discovered later in a backup.

parallels_save_bitmap() already skips an inconsistent bitmap, so it is
not written back. The format has no per bitmap flag to record that the
contents are unusable, so keeping it would present it as valid again on
the next open. Say what happens, as the bitmap disappears from the image
and 'qemu-img bitmap --remove' would report it as missing afterwards.

The image data itself is unaffected: it is repaired at open as before,
and only the bitmaps are dropped.

Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>