efi: Actually enable the ESRT under Xen

The ESRT can be parsed if EFI_PARAVIRT is enabled, even if EFI_MEMMAP is
not.  Also allow the ESRT to be in reclaimable memory, as that is where
future Xen versions will put it.

Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
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>