qemu-img: mandate argument to 'qemu-img check --repair'

qemu-img check --repair option accepts an argument. The argument to
--repair switch can either be 'all' or 'leak'. Fix the long option to
mandate argument with --repair switch.

The patch fixes following segmentation fault

Core was generated by `qemu-img check -f qcow2 --repair all t.qcow2'.
Program terminated with signal 11, Segmentation fault.
0  in img_check (argc=6, argv=0x7fffab9b8a10) at qemu-img.c:588
588	            if (!strcmp(optarg, "leaks")) {
(gdb) bt
  0  img_check (argc=6, argv=0x7fffab9b8a10) at qemu-img.c:588
  1  __libc_start_main () from /lib/x86_64-linux-gnu/libc.so.6
  2  _start ()
(gdb)

Signed-off-by: Prasad Joshi <prasadjoshi.linux@gmail.com>
Reviewed-by: Leandro Dorileo <l@dorileo.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
