SUPPORT.md: extend security support for x86 hosts to 12 TiB of memory

c49ee0329f ("SUPPORT.md: limit security support for hosts with very
much memory"), as a result of XSA-385, restricted security support to
8 TiB of host memory. While subsequently further restricted for Arm,
extend this to 12 TiB on x86, putting in place a guest restriction to
8 TiB (or yet less for Arm) in exchange.

A 12 TiB x86 host was certified successfully for use with Xen 4.14 as
per https://www.suse.com/nbswebapp/yesBulletin.jsp?bulletinNumber=150753.
This in particular included running as many guests (2 TiB each) as
possible in parallel, to actually prove that all the memory can be used
like this. It may be relevant to note that the Optane memory there was
used in memory-only mode, with DRAM acting as cache.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Julien Grall <jgrall@amazon.com>

SUPPORT.md

@@ -50,7 +50,7 @@ For the Cortex A57 r0p0 - r1p1, see Errata 832075.
 
 ### Physical Memory
 
-    Status, x86: Supported up to 8 TiB. Hosts with more memory are supported, but not security supported.
+    Status, x86: Supported up to 12 TiB. Hosts with more memory are supported, but not security supported.
     Status, Arm32: Supported up to 12 GiB
     Status, Arm64: Supported up to 2 TiB
 
@@ -121,6 +121,14 @@ ARM only has one guest type at the moment
 
     Status: Supported
 
+## Guest Limits
+
+### Memory
+
+    Status, x86: Supported up to 8 TiB. Guests with more memory, but less than 16 TiB, are supported, but not security supported.
+    Status, Arm32: Supported up to 12 GiB
+    Status, Arm64: Supported up to 1 TiB
+
 ## Hypervisor file system
 
 ### Build info