Comment # 1 on bug 1183767 from James Fehlig

From the core file

#0  0x00007f10dc546e2c in qemuDomainDefAddDefaultDevices
(qemuCaps=0x7f10d800d2e0, def=0x7f10d8009730)
    at ../../src/qemu/qemu_domain.c:4210
4210            if (STREQ(def->os.machine, "isapc")) {
(gdb) p def->os.machine
$1 = 0x0

The VM config does not explicitly specify a machine, so one should be provided
by virQEMUCapsGetPreferredMachine(), called in qemuDomainDefPostParse().
virQEMUCapsGetPreferredMachine() can return NULL, and commit 67b973b510 will
fix the crash by failing VM creation if os.machine == NULL

https://gitlab.com/libvirt/libvirt/-/commit/67b973b510ad68da06e8eb744d97b3e1df5fbcbe

Is this a regression for you? Were you able to previously create VMs with a
similar configuration? The config (pasted below for easy viewing) worked fine
for me on SLES15 SP2 and Leap 15.2. What qemu packages are installed? Do you
have any capabilities files in /var/cache/libvirt/qemu/capabilities/? If so, do
they list machine types for kvm? E.g. <machine type='kvm' name='pc-i440fx-4.2'
.../>?


VM config extracted from core:

<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
  <name>root_default</name>
  <title></title>
  <description></description>
  <uuid></uuid>
  <memory>524288</memory>
  <vcpu>1</vcpu>
  <cpu mode='host-model'>
    <model fallback='allow'></model>
  </cpu>
  <os>
    <type>hvm</type>
    <kernel></kernel>
    <initrd></initrd>
    <cmdline></cmdline>
  </os>
  <features>
    <acpi/>
    <apic/>
    <pae/>
  </features>
  <clock offset='utc'/>
  <devices>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='default'/>
      <source file='/var/lib/libvirt/images/root_default.img'/>
      <target dev='vda' bus='virtio'/>
    </disk>
    <serial type='pty'>
      <target port='0'/>
    </serial>
    <console type='pty'>
      <target port='0'/>
    </console>
    <input type='mouse' bus='ps2'/>
    <graphics type='vnc' port='-1' autoport='yes' listen='127.0.0.1'
keymap='en-us'/>
    <video>
      <model type='cirrus' vram='9216' heads='1'/>
    </video>
  </devices>
</domain>