SUPPORT.md: Add qemu-depriv section

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>