Hi,
looking at this from qemu's POV.
But it seems you already identified most of the related history, I'll add one 
more (bug 1716028) to the description and then summarize.

** Description changed:

  Juju version: 2.5.0
  Qemu version: qemu-system-x86  2.11+dfsg-1ubuntu7.9
  
  When creating multiple KVM instances using Juju, some fail with "exit
  status 13".
  
  Looking at the unit-machine logs, I can see what seems to be a race-
  condition between qemu-img create and virsh start commands:
  
  2019-02-15 16:31:27 DEBUG juju.container.kvm run_linux.go:24 running: 
qemu-img [create -b /var/lib/juju/kvm/guests/bionic-amd64-backing-file.qcow -f 
qcow2 /var/lib/juju/kvm/guests/juju-720d5c-1-kvm-3.qcow
   8G]
  2019-02-15 16:31:27 DEBUG juju.container.kvm run_linux.go:25 running as uid: 
64055, gid: 117
  2019-02-15 16:31:27 DEBUG juju.container.kvm run_linux.go:36 output: 
Formatting '/var/lib/juju/kvm/guests/juju-720d5c-1-kvm-3.qcow', fmt=qcow2 
size=8589934592 backing_file=/var/lib/juju/kvm/guests/bionic-
  amd64-backing-file.qcow cluster_size=65536 lazy_refcounts=off refcount_bits=16
  
  2019-02-15 16:31:27 DEBUG juju.container.kvm wrappedcmds.go:481 create root 
image: Formatting '/var/lib/juju/kvm/guests/juju-720d5c-1-kvm-3.qcow', 
fmt=qcow2 size=8589934592 backing_file=/var/lib/juju/kvm/
  guests/bionic-amd64-backing-file.qcow cluster_size=65536 lazy_refcounts=off 
refcount_bits=16
  
  2019-02-15 16:31:27 DEBUG juju.container.kvm run.go:19 virsh [define 
/var/lib/juju/containers/juju-720d5c-1-kvm-3/juju-720d5c-1-kvm-3.xml]
  2019-02-15 16:31:27 DEBUG juju.container.kvm run.go:21 output: Domain 
juju-720d5c-1-kvm-3 defined from 
/var/lib/juju/containers/juju-720d5c-1-kvm-3/juju-720d5c-1-kvm-3.xml
  
  2019-02-15 16:31:27 DEBUG juju.container.kvm wrappedcmds.go:204 created
  domain: Domain juju-720d5c-1-kvm-3 defined from /var/lib/juju/containers
  /juju-720d5c-1-kvm-3/juju-720d5c-1-kvm-3.xml
  
  2019-02-15 16:31:27 DEBUG juju.container.kvm run.go:19 virsh [start 
juju-720d5c-1-kvm-3]
  2019-02-15 16:31:29 DEBUG juju.container.kvm run.go:21 output: error: Failed 
to start domain juju-720d5c-1-kvm-3
  error: internal error: qemu unexpectedly closed the monitor: 
2019-02-15T16:31:28.826568Z qemu-system-x86_64: warning: host doesn't support 
requested feature: CPUID.80000001H:ECX.svm [bit 2]
  2019-02-15T16:31:28.839983Z qemu-system-x86_64: -device 
virtio-blk-pci,scsi=off,bus=pci.0,addr=0x3,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1:
 Failed to get shared "write" lock
  Is another process using the image?
  
  2019-02-15 16:31:29 ERROR juju.provisioner.kvm kvm-broker.go:152 failed to 
start container: kvm container creation failed: failed to start domain 
"juju-720d5c-1-kvm-3": exit status 1
  2019-02-15 16:31:29 WARNING juju.provisioner provisioner_task.go:1282 failed 
to start machine 1/kvm/3 (kvm container creation failed: failed to start domain 
"juju-720d5c-1-kvm-3": exit status 1), retrying
   in 10s (10 more attempts)
  
  It never works (on all 10x trials).
  
  There is a lot of reports on qemu-img's request for write lock although 
reading infos:
  https://bugs.launchpad.net/qemu/+bug/1721788
  https://bugs.launchpad.net/nova/+bug/1718295
  https://bugs.launchpad.net/ubuntu/+source/nova/+bug/1718133
+ https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1716028
  
  One solution seems to be passing --force-share to qemu-img command. That
  may be risky in a scenario where Juju AND other things are using qemu at
  the same time, as discussed on:
  https://bugs.launchpad.net/qemu/+bug/1721788/comments/1
  
  Can we create a new image file for every VM to deploy? Or --force-share
  is an alternative?
  
  Either way, there seems to be no work-around besides changing Juju at
  the moment.

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https://bugs.launchpad.net/bugs/1816170

Title:
  KVM instances fail at creation with: Failed to get shared "write" lock
  Is another process using the image?

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