qed: mark image clean after repair succeeds

The dirty bit is cleared after image repair succeeds in qed_open().
Move this into qed_check() so that all callers benefit from this
behavior when fix=true.

This is necessary so qemu-img check can call .bdrv_check() and mark the
image clean.

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