FYI autopkgtest issues all resolved by now.

Started on 1:2.11+dfsg-1ubuntu7.18


...
+ echo 'blockdev-snapshot-sync device=disk0 snapshot-file=tmp.qcow2 format=qcow2'
+ /root/qemu/scripts/qmp/qmp-shell ./qmp-1.sock
Welcome to the QMP low-level shell!
Connected to QEMU 2.11.1

Formatting 'tmp.qcow2', fmt=qcow2 size=10737418240 backing_file=./top-vm01.img backing_fmt=qcow2 cluster_size=65536 lazy_refcounts=off refcount_bits=16
(QEMU) {"return": {}}
(QEMU) 
+ echo 'block-commit device=disk0 base=top-vm01.img'
+ /root/qemu/scripts/qmp/qmp-shell ./qmp-1.sock
Welcome to the QMP low-level shell!
Connected to QEMU 2.11.1
(QEMU) {"error": {"class": "GenericError", "desc": "Failed to get \"write\" lock"}}



Upgraded to proposed and then it works as expected:
root@b:~# ./fulltest 
+ killall qemu-system-x86_64
qemu-system-x86_64: terminating on signal 15 from pid 6666 ()
qemu-system-x86_64: terminating on signal 15 from pid 6666 ()
+ sleep 1
+ rm base.qcow2 tmp.qcow2
+ qemu-img create -f qcow2 base.qcow2 10G
Formatting 'base.qcow2', fmt=qcow2 size=10737418240 cluster_size=65536 lazy_refcounts=off refcount_bits=16
+ qemu-img create -f qcow2 -b base.qcow2 middle-vm01.img 10G
Formatting 'middle-vm01.img', fmt=qcow2 size=10737418240 backing_file=base.qcow2 cluster_size=65536 lazy_refcounts=off refcount_bits=16
+ qemu-img create -f qcow2 -b base.qcow2 middle-vm02.img 10G
Formatting 'middle-vm02.img', fmt=qcow2 size=10737418240 backing_file=base.qcow2 cluster_size=65536 lazy_refcounts=off refcount_bits=16
+ qemu-img create -f qcow2 -b middle-vm01.img top-vm01.img 10G
Formatting 'top-vm01.img', fmt=qcow2 size=10737418240 backing_file=middle-vm01.img cluster_size=65536 lazy_refcounts=off refcount_bits=16
+ qemu-img create -f qcow2 -b middle-vm01.img top-vm02.img 10G
Formatting 'top-vm02.img', fmt=qcow2 size=10737418240 backing_file=middle-vm01.img cluster_size=65536 lazy_refcounts=off refcount_bits=16
+ qemu-system-x86_64 -nographic -qmp unix:./qmp-1.sock,server,nowait -enable-kvm -device virtio-scsi-pci,id=scsi -device sga -nodefaults -monitor none -m 256M -drive file=./top-vm01.img,if=virtio,id=disk0 -smp 1 -smbios type=1,manufacturer=test
+ sleep 2s
+ qemu-system-x86_64 -nographic -qmp unix:./qmp-2.sock,server,nowait -enable-kvm -device virtio-scsi-pci,id=scsi -device sga -nodefaults -monitor none -m 256M -drive file=./top-vm02.img,if=virtio,id=disk0 -smp 1 -smbios type=1,manufacturer=test
qemu-system-x86_64: warning: host doesn't support requested feature: CPUID.80000001H:ECX.svm [bit 2]
qemu-system-x86_64: warning: host doesn't support requested feature: CPUID.80000001H:ECX.svm [bit 2]
+ echo 'blockdev-snapshot-sync device=disk0 snapshot-file=tmp.qcow2 format=qcow2'
+ /root/qemu/scripts/qmp/qmp-shell ./qmp-1.sock
Welcome to the QMP low-level shell!
Connected to QEMU 2.11.1

Formatting 'tmp.qcow2', fmt=qcow2 size=10737418240 backing_file=./top-vm01.img backing_fmt=qcow2 cluster_size=65536 lazy_refcounts=off refcount_bits=16
(QEMU) {"return": {}}
(QEMU) 
+ echo 'block-commit device=disk0 base=top-vm01.img'
+ /root/qemu/scripts/qmp/qmp-shell ./qmp-1.sock
Welcome to the QMP low-level shell!
Connected to QEMU 2.11.1

(QEMU) {"return": {}}
(QEMU) 


Version used was 1:2.11+dfsg-1ubuntu7.19