xen: enable platform-pci only in a Xen guest

While debugging kexec issues in a PVonHVM guest I modified
xen_hvm_platform() to return false to disable all PV drivers. This
caused a crash in platform_pci_init() because it expects certain data
structures to be initialized properly.

To avoid such a crash make sure the driver is initialized only if
running in a Xen guest.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>