[gnome-build-meta.wiki] Update Bootable images in virtual machines - add some debugging tips

[Sam Thursfield <sthursfield src gnome org>]

commit 3bc21fc729f578ffe7646074dbb58adefac36daf
Author: Sam Thursfield <sam afuera me uk>
Date:   Wed Aug 11 12:34:54 2021 +0000

    Update Bootable images in virtual machines - add some debugging tips

 Bootable-images-in-virtual-machines.md | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/Bootable-images-in-virtual-machines.md b/Bootable-images-in-virtual-machines.md
index c70cb35..8f4e04f 100644
--- a/Bootable-images-in-virtual-machines.md
+++ b/Bootable-images-in-virtual-machines.md
@@ -192,6 +192,14 @@ Then you can `spice+unix://${XDG_RUNTIME_DIR:-/tmp}/vm.sock`
 
 ## Appendix
 
+### Debugging a failed boot
+
+The bootloader is [systemd-boot](https://www.freedesktop.org/wiki/Software/systemd/systemd-boot/), you can 
press `e` at the menu to edit kernel commandline arguments. If you get a black screen on boot, try  removing 
`quiet` and `splash` arguments.
+
+Set `console=ttyS0` kernel argument to enable login on the serial console. If running `qemu` from a 
terminal, you can pass `-serial stdio` to QEMU to connect serial console to your terminal, which allows you 
to copy and paste.
+
+There is no root password set by default. You can get a login shell by adding kernel argument 
`systemd.debug-console=1`, then switching to VT9 by pressing `<ctrl>+<alt>+<F9>`.
+
 ### nr_entries is too big
 
 GNOME Shell seems to hit a limit in QEMU. If you get this error message