common/grant_table: harden helpers

common/grant_table: harden helpers

Guests can issue grant table operations and provide guest controlled
data to them. This data is used for memory loads in helper functions
and macros. To avoid speculative out-of-bound accesses, we use the
array_index_nospec macro where applicable, or the block_speculation
macro.

This is part of the speculative hardening effort.

Signed-off-by: Norbert Manthey <nmanthey@amazon.de>
Reviewed-by: Jan Beulich <jbeulich@suse.com>