xen/arm: io: Harden sign extension check

xen/arm: io: Harden sign extension check

In the ideal world we would never get an undefined behavior when
propagating the sign bit since that bit can only be set for access
size smaller than the register size (i.e byte/half-word for aarch32,
byte/half-word/word for aarch64).

In the real world we need to care for *possible* hardware bug such as
advertising a sign extension for either 64-bit (or 32-bit) on Arm64
(resp. Arm32).

So harden a bit more the code to prevent undefined behavior when
propagating the sign bit in case of buggy hardware.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
CC: Julien Grall <julien.grall@arm.com>