xen/arm: traps: Second attempt to correctly use the content of HPFAR_EL2

xen/arm: traps: Second attempt to correctly use the content of HPFAR_EL2

Commit c051618 "xen/arm: traps: Correctly interpret the content of the
register HPFAR_EL2" attempted to fix the interpretation of HPFAR_EL2.

However, the register contains a 4KB-aligned address. This means that
the reported address is not directly usable to know the faulting IPA.
The offset in the 4KB page can be found by looking at the associated virtual
address (FAR_EL2/HDFAR).

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>