efi: Apply allowlist to EFI configuration tables when running under Xen

As it turns out, Xen does not guarantee that EFI boot services data
regions in memory are preserved, which means that EFI configuration
tables pointing into such memory regions may be corrupted before the
dom0 OS has had a chance to inspect them.

This is causing problems for Qubes OS when it attempts to perform system
firmware updates, which requires that the contents of the EFI System
Resource Table are valid when the fwupd userspace program runs.

However, other configuration tables such as the memory attributes table
or the runtime properties table are equally affected, and so we need a
comprehensive workaround that works for any table type.

So when running under Xen, check the EFI memory descriptor covering the
start of the table, and disregard the table if it does not reside in
memory that is preserved by Xen.

Co-developed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>