Move hardcoded initialization of xen-platform device.

Creation of the xen-platform device is currently hardcoded into machine
type pc's initialization code, guarded by a test for the whether the xen
accelerator is enabled. This patch moves the creation of xen-platform into
the initialization code of the xenfv machine type. This maintains backwards
compatibility for that machine type but allows more flexibility if another
machine type is used with Xen HVM domains.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
