feat: add QEMU software emulation fallback for VM testing

VM creation failed with "Emulator does not support virt type 'kvm'"
in environments without /dev/kvm (nested VMs, CI, containers). This
makes ISO boot testing work everywhere by detecting KVM at runtime.

Changes:
- vm/template.xml: replace hardcoded type='kvm' and cpu
  mode='host-passthrough' with @DOMAIN_TYPE@ and @CPU_MODE@ placeholders
- run.sh vm_create(): detect /dev/kvm at runtime, use KVM +
  host-passthrough when available, fall back to QEMU + qemu64
  software emulation when not
- tests: 6 new tests covering placeholder existence, KVM detection,
  and substitution logic

This enables the ISO to be booted and serial output captured for
verification even without hardware virtualization support.

💘 Generated with Crush

Assisted-by: GLM-4.7 via Crush <crush@charm.land>

vm/template.xml

@@ -1,4 +1,4 @@
-<domain type='kvm'>
+<domain type='@DOMAIN_TYPE@'>
   <name>@VM_NAME@</name>
   <uuid>@VM_UUID@</uuid>
   <memory unit='MiB'>@VM_RAM@</memory>
@@ -16,7 +16,7 @@
     <apic/>
     <smm state='on'/>
   </features>
-  <cpu mode='host-passthrough' check='none'/>
+  <cpu mode='@CPU_MODE@' check='none'/>
   <clock offset='utc'>
     <timer name='rtc' tickpolicy='catchup'/>
     <timer name='pit' tickpolicy='delay'/>