SUPPORT.md: Add qemu-depriv section

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
---
Changes since v4:
- Fix some grammar (s/attack/attacking/;)

Changes since v3:
- Moved from the qemu-depriv doc patches.
- Reword to include the possibility of having a non-dom0 "devicemodel"
  domain which may want to be protected
- Specify `Linux dom0` as the currently-tech-supported window

CC: Ian Jackson <ian.jackson@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Jan Beulich <jbeulich@suse.com>
CC: Tim Deegan <tim@xen.org>
CC: Konrad Wilk <konrad.wilk@oracle.com>
CC: Stefano Stabellini <sstabellini@kernel.org>
CC: Julien Grall <julien.grall@arm.com>
CC: Anthony Perard <anthony.perard@citrix.com>
CC: Ross Lagerwall <ross.lagerwall@citrix.com>

SUPPORT.md

@@ -526,6 +526,26 @@ Vulnerabilities of a device model stub domain
 to a hostile driver domain (either compromised or untrusted)
 are excluded from security support.
 
+### Device Model Deprivileging
+
+    Status, Linux dom0: Tech Preview, with limited support
+
+This means adding extra restrictions to a device model in order to
+prevent a compromised device model from attacking the rest of the
+domain it's running in (normally dom0).
+
+"Tech preview with limited support" means we will not issue XSAs for
+the _additional_ functionality provided by the feature; but we will
+issue XSAs in the event that enabling this feature opens up a security
+hole that would not be present without the feature disabled.
+
+For example, while this is classified as tech preview, a bug in libxl
+which failed to change the user ID of QEMU would not receive an XSA,
+since without this feature the user ID wouldn't be changed. But a
+change which made it possible for a compromised guest to read
+arbitrary files on the host filesystem without compromising QEMU would
+be issued an XSA, since that does weaken security.
+
 ### KCONFIG Expert
 
     Status: Experimental