qemu 6.1 has decided to change qemu-img create so that a backing
format (-F) is required if a backing file (-b) is specified. Since we
don't want to change the libguestfs API to force callers to specify
this because that would be an API break, autodetect it.
This is similar to commit c8c181e8d9 ("launch: libvirt: Autodetect
backing format for readonly drive overlays").
Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1998820
(cherry picked from commit 45de287447)