qed: Images with backing file do not require QED_F_NEED_CHECK

The consistency check on open is necessary in order to fix inconsistent
table offsets left as a result of a crash mid-operation.  Images with a
backing file actually flush before updating table offsets and are
therefore guaranteed to be consistent.  Do not mark these images dirty.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
