installation-upgrade.md : Add a troubleshooting entry for upgrade not offered (UEFI vs BIOS)

Author: yblatti

docs/troubleshooting/installation-upgrade.md

@@ -29,6 +29,35 @@ When asking for help about installation errors, providing this file increases yo
 
 The target installation partition is mounted in `/tmp/root`.
 
+## The ISO installer does not offer to upgrade the existing install (XCP-ng or XenServer)
+
+For the installer to detect your current install (XCP-ng or XenServer), the ISO must be booted in the same firmware context.\
+The difference is how you pick the drive at boot. This applies to physical and virtual CD/DVD/USB drives.
+
+If your existing installation is in legacy BIOS mode, boot the ISO is BIOS mode.
+
+If your existing installation is in UEFI mode, boot the ISO is UEFI mode.
+
+In your server boot menu, you might see something like this : 
+
+```
+Please select boot device:
+---------------------------------
+UEFI: Virtual CDROM Device   <<<<<< This one is in UEFI mode
+Virtual CDROM Device         <<<<<< This one is the same device, in legacy BIOS mode
+...
+ other boot entries
+...
+---------------------------------
+```
+
+### How identify if a running install is using UEFI or legacy BIOS ?
+
+On the host, run `efibootmgr`
+
+- If ou see `EFI variables are not supported on this system.` you're running on Legacy BIOS
+- If you see some EFI boot entries, you’re running on UEFI
+
 ## Installation logs
 
 The installer writes in `/var/log/installer/`.