Dear Maintainer,

None of the current x86_64 firmware include the "acpi-s4" feature,
preventing configuration of:

  <pm>
    <suspend-to-disk enabled='yes'/>
  </pm>

in any libvirt domain using UEFI firmware, which in turn prevents the
ability to run "virsh dompmsuspend <domain> disk" to ask the domain to
hibernate.

According to the following, "acpi-s4" may be safe to use:

https://edk2.groups.io/g/devel/topic/the_status_of_acpi_s4_support/82204810

I have tried adding the feature to a custom firmware based on
/usr/share/qemu/firmware/60-edk2-x86_64.json and it seems to work.

Please investigate and consider adding the "acpi-s4" feature to the
provided x86_64 firmware.

Thanks.