xen: arm: check permissions when copying to/from guest virtual addresses

xen: arm: check permissions when copying to/from guest virtual addresses

In particular we need to make sure the guest has write permissions to buffers
which it passes as output buffers for hypercalls, otherwise the guest can
overwrite memory which it shouldn't be able to write (like r/o grant table
mappings).

This is XSA-98.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>