Description of problem:
From https://qemu.weilnetz.de/doc/qemu-doc.html#qemu_005fimg_005finvocation, we get info "If a relative path name is given, the backing file is looked up relative to the directory containing output_filename.". However, while practising this over RBD, it does not work now. 

Version-Release number of selected component (if applicable):
qemu-kvm-4.1.0-4.module+el8.1.0+4020+16089f93
kernel-4.18.0-135.el8


How reproducible:
100%


Steps to Reproduce:
1. Create a base image locally
# qemu-img create -f qcow2 base.qcow2 2G

2. Create overlay over RBD based on base.qcow2
# qemu-img create -f qcow2 -b base.qcow2 rbd:kvmtest-pool/sn.qcow2

# qemu-img info rbd:kvmtest-pool/sn.qcow2
image: json:{"driver": "qcow2", "file": {"pool": "kvmtest-pool", "image": "sn.qcow2", "driver": "rbd"}}
file format: qcow2
virtual size: 20 GiB (21474836480 bytes)
disk size: unavailable
cluster_size: 65536
backing file: base.qcow2 (cannot determine actual path)
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false


Actual results:
As above, created with no error.


Expected results:
Create image fails, and give info like below:

qemu-img: rbd:kvmtest-pool/sn.qcow2: Could not open 'rbd:kvmtest-pool/base.qcow2': No such file or directory


Additional info:
Doing more tests(Check/rebase) will failed.
# qemu-img check rbd:kvmtest-pool/sn.qcow2
qemu-img: Could not open 'rbd:kvmtest-pool/sn.qcow2': Cannot generate a base directory for rbd nodes

# qemu-img rebase -b /home/test/ceph_qcow2/new_base.qcow2 rbd:kvmtest-pool/sn.qcow2 -p
qemu-img: Could not open 'rbd:kvmtest-pool/sn.qcow2': Cannot generate a base directory for rbd nodes