xen/arm: p2m: Add more debug in get_page_from_gva

xen/arm: p2m: Add more debug in get_page_from_gva

The function get_page_from_gva is used by copy_*_guest helpers to
translate a guest virtual address to a machine physical address and take
reference on the page.

There are a couple of errors paths that will return the same value making
it difficult to know the exact error. Add more debug in each error patch
only for debug-build.

This should help narrowing down the intermittent failure with the
hypercall GNTTABOP_copy (see [1]).

[1] https://lists.xen.org/archives/html/xen-devel/2017-11/msg00942.html

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>