xen/arm: Remove most of the *_VIRT_END defines

xen/arm: Remove most of the *_VIRT_END defines

At the moment, *_VIRT_END may either point to the address after the end
or the last address of the region.

The lack of consistency make quite difficult to reason with them.

Furthermore, there is a risk of overflow in the case where the address
points past to the end. I am not aware of any cases, so this is only a
latent bug.

Start to solve the problem by removing all the *_VIRT_END exclusively used
by the Arm code and add *_VIRT_SIZE when it is not present.

Take the opportunity to rename BOOT_FDT_SLOT_SIZE to BOOT_FDT_VIRT_SIZE
for better consistency and use _AT(vaddr_t, ).

Also take the opportunity to fix the coding style of the comment touched
in mm.c.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Tested-By: Luca Fancellu <luca.fancellu@arm.com>