hw/xen: Automatically add xen-platform PCI device for emulated Xen guests

hw/xen: Automatically add xen-platform PCI device for emulated Xen guests

It isn't strictly mandatory but Linux guests at least will only map their
grant tables over the dummy BAR that it provides, and don't sufficient wit
to map them in any other unused part of their guest address space. So
include it by default for minimal surprise factor.

As I come to document "how to run a Xen guest in QEMU", this means one
fewer thing to tell the user about, according to the mantra of "if it
needs documenting, fix it first, then document what remains".

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>