bug: Secure Boot does not work for Linux guests

[alexhaydock]

I confirm this bug has not already been reported

• I have searched the issues and this bug has not been reported previously

Describe the bug
When booting Linux guests, enabling Secure Boot with secureboot="on" appears to stop the VM from booting.

I have tested this on Linux hosts, but not on any other host platforms so mileage may vary there.

This is partially related to my issue #413.

PR to follow with some suggested steps to solve this, though it may need some work. The PR should also partially fix #413.

To Reproduce
Steps to reproduce the behaviour:
Run a Quickemu guest with the following .conf (example uses Fedora but it works for all Linux distros):

#!/usr/bin/quickemu --vm
guest_os="linux"
bios_type="efi"
secureboot="on"
disk_img="test/disk.qcow2"
iso="test/Fedora-Workstation-Live-x86_64-41-1.4.iso"

Experience the fact that the VM does not provide any display output:

Expected behaviour
VM should boot any Linux guest that has Secure Boot support (Debian, Fedora, Ubuntu, etc) without issues, and report that Secure Boot is Enabled when running mokutil --sb-state (which indicates that Secure Boot is not only working, but that the Microsoft Platform Keys are enabled as per #413).

System information
Running latest Quickemu from master on Fedora 41 Workstation.

Additional context
PR to follow.