A deprecated feature of qemu-img using backing file without backing format

Issue

qemu-img create command shows the following warning unless specifying a backing format using -F option or -o backing_format= option since RHEL9:

    # qemu-img create -f qcow2 base.qcow2 100M
    Formatting 'base.qcow2', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=104857600 lazy_refcounts=off refcount_bits=16
    # qemu-img create -f qcow2 -b base.qcow2 diff.qcow2 100M
    qemu-img: warning: Deprecated use of backing file without explicit backing format (detected format of qcow2)
    Formatting 'diff.qcow2', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=104857600 backing_file=base.qcow2 backing_fmt=qcow2 lazy_refcounts=off refcount_bits=16

    # qemu-img create -f raw base.img 100M
    Formatting 'base.img', fmt=raw size=104857600
    # qemu-img create -f qcow2 -b base.img diff2.qcow2 100M
    qemu-img: warning: Deprecated use of backing file without explicit backing format (detected format of raw)
    Formatting 'diff2.qcow2', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=104857600 backing_file=base.img lazy_refcounts=off refcount_bits=16