exec: Limit translation limiting in address_space_translate to xen

The address_space_translate() function cuts the returned plen (page size)
to hardcoded TARGET_PAGE_SIZE. This function can be used on pages bigger
than that so this limiting should not be used on such pages.

Since originally the limiting was introduced for XEN, we can safely
limit this piece of code to XEN. So does the patch.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
