qemu-img: add json output option to the check command

This option --output=[human|json] makes qemu-img check output a human
or JSON representation at the choice of the user.

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>