qcow2: Remove dead check on !ret

In the beginning of the function, we initialize the local variable to 0,
and in the body of the function, we check the assigned values and exit
the loop immediately. So here it can never be non-zero.

Reported-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>