xen/arm: xen/arm: Reinforce use of is_hardware_domain

xen/arm: xen/arm: Reinforce use of is_hardware_domain

There are a few places on Arm where we use pretty much an open-coded
version of is_hardware_domain(). The main difference, is the helper
will also block speculation (not yet implemented on Arm).

The existing users are not in hot path, so blocking speculation
would not hurt when it is implemented. So remove the open-coded
version within the arm codebase.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
[julieng: Rework the commit message]
Acked-by: Julien Grall <jgrall@amazon.com>