test(instances): live tests with a running guest OS

The live tests so far ran VMs without an OS, so everything that needs the
guest's cooperation was untested. With TEST_LIBVIRT_GUEST_IMAGE set to an
installed Linux image with qemu-guest-agent, instances/test_live_guest.py
boots wvc-test VMs from qcow2 overlays of it (the image is only read) and
checks through the web views:

- delete_vol on a running VM: the guest releases the disk, so it leaves
  the live and persistent definitions and the volume is deleted;
- detach_vol removes the disk from the running guest and keeps the volume;
- add_network/delete_network: the guest sees the hotplugged NIC, and the
  unplug completes in both definitions;
- set_vcpu brings a hotplugged vCPU online in the guest;
- osinfo returns the guest agent's OS, time zone and host name, and the
  agent reports the VM's NIC by MAC;
- poweroff shuts the guest down through libvirt.

The image must be a BIOS-bootable qcow2 volume in a storage pool other
than wvc-test; the tests are skipped while a running VM uses it. They pass
against a Rocky Linux 9 guest on a RHEL qemu-kvm host; the guest had no
address on the NAT network (its network configuration is static), so only
the agent's NIC report is checked, not an address.

README.md

@@ -460,8 +460,10 @@ ruff check .
 > ```bash
 > export TEST_LIBVIRT_HOST=compute1 TEST_LIBVIRT_TYPE=2   # 1 TCP, 2 SSH, 3 TLS, 4 socket
 > export TEST_LIBVIRT_LOGIN=root TEST_LIBVIRT_PASSWORD=   # login/password for TCP and TLS
+> export TEST_LIBVIRT_GUEST_IMAGE=/var/lib/libvirt/images/linux.qcow2   # optional, see below
 > python manage.py test
 > ```
+> `TEST_LIBVIRT_GUEST_IMAGE` enables the tests that need a guest OS (disk and NIC hot-unplug, vCPU hotplug, guest agent, ACPI power off). It must be an installed, BIOS-bootable Linux qcow2 image with qemu-guest-agent, in a storage pool of the host other than `wvc-test`; the tests boot copies of it from overlays and only read the image. Shut down any VM using it first.
 
 ## Users, Roles and Permissions