ACPI: fix return value of XEN_PM_PDC platform op

Should return -EFAULT when copying to guest memory fails.

Once touching this code, also switch to using the more relaxed copy
function (copying from the same guest memory already validated the
virtual address range).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/drivers/acpi/pmstat.c
+++ b/xen/drivers/acpi/pmstat.c
@@ -521,8 +521,8 @@ int acpi_set_pdc_bits(u32 acpi_id, XEN_G
                     ACPI_PDC_SMP_C1PT) & ~mask;
         ret = arch_acpi_set_pdc_bits(acpi_id, bits, mask);
     }
-    if ( !ret )
-        ret = copy_to_guest_offset(pdc, 2, bits + 2, 1);
+    if ( !ret && __copy_to_guest_offset(pdc, 2, bits + 2, 1) )
+        ret = -EFAULT;
 
     return ret;
 }