'?' for help is deprecated since commit c8057f95 "Support 'help' as
a synonym for '?' in command line options", v1.2.0.  We neglected to
update output of qemu-img --help and the manual.  Do that now.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20220908130842.641410-1-armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>