SUPPORT.md: make Linux based stubdom fully supported

All patches needed for running with a Linux stubdom device model are
in the tree and QubesOS is using and testing Linux stubdoms nowadays.

Switch support from "Tech Preview" to "Supported, with caveats".

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-By: Oleksii Kurochko<oleksii.kurochko@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

CHANGELOG.md

@@ -10,6 +10,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
  - The minimum toolchain requirements have increased for some architectures:
    - For x86, GCC 5.1 and Binutils 2.25, or Clang/LLVM 11
    - For ARM32 and ARM64, GCC 5.1 and Binutils 2.25
+ - Linux based device model stubdomains are now fully supported.
 
 ### Added
  - On x86:

SUPPORT.md

@@ -260,7 +260,10 @@ Go (golang) bindings for libxl
 
 Support for running qemu-xen device model in a linux stubdomain.
 
-    Status: Tech Preview
+    Status: Supported, with caveats
+
+Any issue in the stubdomain affecting only the guest it is servicing
+or itself will not be regarded a security issue.
 
 ## Xenstore