xen/pci: gate APEI support on ARM

xen/pci: gate APEI support on ARM

APEI not supported on ARM yet move the code under CONFIG_X86 flag to
gate the code for ARM.

This patch is the preparatory work to enable HAS_PCI on ARM to avoid
compilation error on ARM.

prelink.o: In function `pcie_aer_get_firmware_first’:
drivers/passthrough/pci.c:1251: undefined reference to `apei_hest_parse'

Signed-off-by: Rahul Singh <rahul.singh@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>