xen/pci: move x86 specific code to x86 directory

xen/pci: move x86 specific code to x86 directory

passthrough/pci.c file is common for all architecture, but there is x86
specific code in this file.

Move x86 specific code to the drivers/passthrough/io.c file to avoid
compilation error for other architecture.

As drivers/passthrough/io.c is compiled only for x86 move it to
x86 directory and rename it to hvm.c.

No functional change intended.

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