qemu-img: Document backing options

The create and convert subcommands have shorthands to set the
backing_file and, in the case of create, the backing_fmt options for the
new image. However, they have not been documented so far, which is
remedied by this patch.

Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>