Since the following libvirt commit:
commit c9b13e05570d07addb4bfb86c5baf373064842e0

    qemu: Use JSON directly for '-device'

git describe: v7.8.0-225-gc9b13e0557 contains: v7.9.0-rc1~82

We only receive the DEVICE_DELETED event for the backend:
{
  "timestamp": {
    "seconds": 1640268422,
    "microseconds": 765168
  },
  "event": "DEVICE_DELETED",
  "data": {
    "path": "/machine/peripheral/virtio-disk2/virtio-backend"
  }
}

Before, we would get one for the frontend as well:

{
  "timestamp": {
    "seconds": 1640268422,
    "microseconds": 815923
  },
  "event": "DEVICE_DELETED",
  "data": {
    "device": "virtio-disk2",
    "path": "/machine/peripheral/virtio-disk2"
  }
}

I've filed https://bugzilla.redhat.com/show_bug.cgi?id=2036669 to track the qemu regression of not sending DEVICE_DELETED for the frontend when JSON syntax is used for -device.

For now libvirt will need to revert the support for JSON with -device until qemu is fixed. This bug will track the reversion.

*** Bug 2037765 has been marked as a duplicate of this bug. ***

Upstream works this around by disabling the use of JSON with -device for now until qemu fixes it:

commit bd3d00babc9b9b51bf2ee3ee39fafe479b8f8ae3
Author: Peter Krempa <pkrempa>
Date:   Mon Jan 3 15:50:49 2022 +0100

    qemu: Revert to using non-JSON commandline for -device
    
    When -device is configured via JSON a bug [1] is triggered in qemu were
    the DEVICE_DELETED event for the removal of the device frontend is no
    longer delivered to libvirt. Without the DEVICE_DELETED event we don't
    remove the corresponding entries in the VM XML.
    
    Until qemu will be fixed we must stop using the JSON syntax for -device.
    
    This patch removes the detection of the capability. The capability is
    used only during startup of a fresh VM so we don't need to consider any
    compaitibility steps for existing VMs.
    
    For users who wish to use 'libvirt-7.9' and 'libvirt-7.10' with
    'qemu-6.2' there are two possible workarounds:
    
     - filter out the 'device.json' qemu capability '/etc/libvirt/qemu.conf':
    
       capability_filters = [ "device.json" ]
    
     - filter out the 'device.json' qemu capability via qemu namespace XML:
    
       <domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
         [...]
         <qemu:capabilities>
           <qemu:del capability='device.json'/>
         </qemu:capabilities>
       </domain>
    
    We must never again use the same capability name as we are now
    instructing users to filter it as a workaround so once qemu is fixed
    we'll need to pick a new capability value for it.
    
    [1] https://bugzilla.redhat.com/show_bug.cgi?id=2036669
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2035237
    Signed-off-by: Peter Krempa <pkrempa>
    Reviewed-by: Ani Sinha <ani>
    Reviewed-by: Ján Tomko <jtomko>

v7.10.0-470-gbd3d00babc

(Note that the workaround described in the commit message is _NOT_ needed with libvirt-8.0, that's just for fixing older versions.)

Verified Version:
libvirt-8.0.0-1.module+el8.6.0+13888+55157bfb.x86_64
qemu-kvm-6.2.0-2.module+el8.6.0+13738+17338784.x86_64
kernel-4.18.0-359.el8.x86_64

Verified Steps:
S1: Hot-unplug disk from guest.
1. Prepare a guest and a disk image.
# ll /var/lib/libvirt/images/test.img 
-rw-r--r--. 1 root root 209715200 Jan 17 22:19 /var/lib/libvirt/images/test.img
# virsh domstate lmn
shut off
2. Attach a disk with --config.
# virsh attach-disk --domain lmn --source /var/lib/libvirt/images/test.img --target vdb --driver qemu --config
Disk attached successfully
3. Start the guest.
# virsh start lmn
Domain 'lmn' started
# virsh domblklist lmn
 Target   Source
---------------------------------------------
 vda      /var/lib/libvirt/images/lmn.qcow2
 vdb      /var/lib/libvirt/images/test.img
4. Check the dumpxml.
# virsh dumpxml lmn | grep /disk -B7
......
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/test.img' index='1'/>
      <backingStore/>
      <target dev='vdb' bus='virtio'/>
      <alias name='virtio-disk1'/>
      <address type='pci' domain='0x0000' bus='0x07' slot='0x00' function='0x0'/>
    </disk>
5. Detach the disk.
# virsh detach-disk lmn vdb
Disk detached successfully
# virsh dumpxml lmn | grep /disk -B7
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/lmn.qcow2' index='2'/>
      <backingStore/>
      <target dev='vda' bus='virtio'/>
      <alias name='virtio-disk0'/>
      <address type='pci' domain='0x0000' bus='0x04' slot='0x00' function='0x0'/>
    </disk>
S2: Check the related jobs of features in this bug, and find no related qemu cmd issues.

*** Bug 2035006 has been marked as a duplicate of this bug. ***

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (Moderate: virt:rhel and virt-devel:rhel security, bug fix, and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2022:1759