vagrant-libvirt's chosen CPU cannot be emulated on Intel Core i7-4600U

==> default: Starting domain. There was an error talking to Libvirt. The error message is shown below:

Call to virDomainCreateWithFlags failed: internal error: early end of file from monitor, possible problem: warning: host doesn't support requested feature: CPUID.01H:EDX.ds [bit 21]
warning: host doesn't support requested feature: CPUID.01H:EDX.acpi [bit 22]
warning: host doesn't support requested feature: CPUID.01H:EDX.ht [bit 28]
warning: host doesn't support requested feature: CPUID.01H:EDX.tm [bit 29]
warning: host doesn't support requested feature: CPUID.01H:EDX.pbe [bit 31]
warning: host doesn't support requested feature: CPUID.01H:ECX.dtes64 [bit 2]
warning: host doesn't support requested feature: CPUID.01H:ECX.monitor [bit 3]
warning: host doesn't support requested feature: CPUID.01H:ECX.ds_cpl [bit 4]
warning: host doesn't support requested feature: CPUID.01H:ECX.smx [bit 6]
warning: host doesn't support requested feature: CPUID.01H:ECX.est [bit 7]
warning: host doesn't support requested feature: CPUID.01H:ECX.tm2 [bit 8]
warning: host doesn't support requested feature: CPUID.01H:ECX.xtpr [bit 14]
warning: host doesn't support requested feature: CPUID.01
rake aborted!

And the domain’s XML says:

  <cpu mode='host-model'>
    <model fallback='allow'>qemu64</model>
  </cpu>

… which actually is not really valid, btw: https://libvirt.org/formatdomain.html#elementsCPU says that “Specifying CPU model is not supported either”, about host-model.

This fixes it for me:

--- a/vagrant/Vagrantfile
+++ b/vagrant/Vagrantfile
@@ -37,6 +37,7 @@ Vagrant.configure("2") do |config|
     domain.connect_via_ssh = false
     domain.machine_arch = 'x86_64'
     domain.machine_type = 'q35'
+    domain.cpu_mode = 'host-passthrough'
     domain.emulator_path = '/usr/bin/qemu-system-x86_64'
     domain.memory = ENV['TAILS_RAM_BUILD'] ? VM_MEMORY_FOR_RAM_BUILDS :
                                              VM_MEMORY_FOR_DISK_BUILDS

Feature Branch: bugfix/11410-vagrant-libvirt-cpu-mode

Parent Task: #7526 (closed)

Original created by @intrigeri on 11410 (Redmine)