qemu-img: fix failed autotests

There are 9 iotests failed on Ubuntu 15.10 at the moment.
The problem is that options parsing in qemu-img is broken by the
following commit:
    commit 10985131e337a0c52c5bd1e191fd7867a6ff8d02
    Author: Denis V. Lunev <den@openvz.org>
    Date:   Fri Jun 17 17:44:13 2016 +0300
    qemu-img: move common options parsing before commands processing

This strange command line reports error
  ./qemu-img create -f qcow2 TEST_DIR/t.qcow2 -- 1024
  qemu-img: Invalid image size specified!
while original code parses it successfully.

The problem is that getopt_long state should be reset. This could be done
using this assignment according to the manual:
    optind = 0

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
