qemu-io: Add missing option documentation

The Usage: summary is missing several options, but rather than
having to maintain it, it's simpler to just state [OPTIONS],
since the options are spelled out below.

Commit 499afa2 added --image-opts, but forgot to document it in
--help.  Likewise for commit 9e8f183 and -d/--discard.

Commit e3aff4f6 put "-o/--offset" in the long opts, but it has
never been honored.

Add a note that '-n' is short for '-t none'.

Commit 9a2d77ad killed the -C option, but forgot to undocument
it for the 'open' subcommand.

Finally, commit 10d9d75 removed -g/--growable, but forgot to
cull it from the valid short options.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-id: 1462677405-4752-2-git-send-email-eblake@redhat.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
