qcow2: Flushing the caches in qcow2_close may fail

qcow2: Flushing the caches in qcow2_close may fail

qcow2_cache_flush() may fail; if one of the caches failed to be flushed
successfully to disk in qcow2_close() the image should not be marked
clean, and we should emit a warning.

This breaks the (qcow2-specific) iotests 026, 071 and 089; change their
output accordingly.

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 3b5e14c76a6bb142bf250ddf99e24a0ac8c7bc12)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>