Description of problem:
libvirt gives base image (as the argument) to qemu while starting VM

Version-Release number of selected component (if applicable):
5.10.0-alt1

How reproducible:
ALWAYS

Steps to Reproduce:
1. qemu-img create -f qcow2 1.img 100M
2. qemu-img create -f qcow2 -b 1.img 2.img
3. create VM with 2.img as a disk (IDE by default, created with virt-manager)
4. in the VM do "hexdump -C /dev/sda | head"
5. see "QFI...", i.e. the contents of 1.img

Actual results:
/dev/sda's contents is "QFI...", the contents of 1.img
Also the 2.img doesn't seem to show in the VM

Expected results:
zeroes

Additional info:
looking into `ps` command output I see that qemu cmdline is weird, containing 1.img as raw image:
> /usr/bin/qemu-system-x86_64 \
  -name guest=altlinux7,debug-threads=on
...
  -blockdev {"driver":"file","filename":"/.../tmp/1.img","node-name":"libvirt-3-storage","auto-read-only":true,"discard":"unmap"}
  -blockdev {"node-name":"libvirt-3-format","read-only":true,"driver":"raw","file":"libvirt-3-storage"}
  -blockdev {"driver":"file","filename":"/.../tmp/2.img","node-name":"libvirt-2-storage","auto-read-only":true,"discard":"unmap"}
  -blockdev {"node-name":"libvirt-2-format","read-only":false,"driver":"qcow2","file":"libvirt-2-storage","backing":"libvirt-3-format"}
...