vm: note on skip forced disk controllers uefi secure boot (#616)

* vm: note on skip forced disk controllers uefi secure boot

Related apache/cloudstack#11750

Signed-off-by: Abhishek Kumar <abhishek.mrt22@gmail.com>

* Apply suggestion from @shwstppr

* Apply suggestions from code review

Co-authored-by: Suresh Kumar Anaparti <sureshkumar.anaparti@gmail.com>

---------

Signed-off-by: Abhishek Kumar <abhishek.mrt22@gmail.com>
Co-authored-by: Suresh Kumar Anaparti <sureshkumar.anaparti@gmail.com>

Author: shwstppr

source/adminguide/virtual_machines.rst

@@ -175,6 +175,18 @@ To create an Instance from a Template:
    specified at the Instance or Template level. For an existing Instance its settings can be updated while it is in
    stopped state by admin.
 
+   **KVM**
+
+   Instances running on the KVM hypervisor with UEFI Secure Boot have disk controllers automatically enforced as following:
+
+   - Windows OS instances use SATA
+   - Non-Windows OS instances use VirtIO
+
+   Starting with 4.20.3 and later, this behavior can be overridden by setting the following template or instance detail to true:
+
+   ``skip.force.disk.controller = true``
+
+   When set to true, disk controller enforcement is skipped, and the controllers defined by template/instance details are used. If the detail is added at both template and instance level, the instance detail takes precedence.
 
 Install Required Tools and Drivers
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~