SUPPORT.md: Desupport qemu trad except stub dm

SUPPORT.md: Desupport qemu trad except stub dm

While investigating XSA-335 we discovered that many upstream security
fixes were missing.  It is not practical to backport them.  There is
no good reason to be running this very ancient version of qemu, except
that it is the only way to run a stub dm which is currently supported
by upstream.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
master commit: 8587160b3e2951b722d395a0346bb17c3c22152f
master date: 2020-11-04 09:22:37 +0100