qemu-img: rebase: refresh options/--help (short option change)

qemu-img: rebase: refresh options/--help (short option change)

Add missing long options and --help output,
reorder options for consistency.

Use -B for --backing-format, keep -F for
backwards compatibility.

Options added:
 --format, --cache - for the image in question
 --backing, --backing-format, --backing-cache, --backing-unsafe -
   for the new backing file
(was eg CACHE vs SRC_CACHE, which is unclear).

Probably should rename local variables.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Message-ID: <20250531171609.197078-19-mjt@tls.msk.ru>
[kwolf: Removed command description from the argument list]
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>