Arm now can use the "dom0=" Xen command line option and the support
for guests running SVE instructions is added, put entries in the
changelog.

Mention the "Tech Preview" status and add an entry in SUPPORT.md

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
Acked-by: Henry Wang <Henry.Wang@arm.com> # CHANGELOG
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>