hw/xen: detect when running inside stubdomain

hw/xen: detect when running inside stubdomain

Introduce global xen_is_stubdomain variable when qemu is running inside
a stubdomain instead of dom0. This will be relevant for subsequent
patches, as few things like accessing PCI config space need to be done
differently.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <e66aa97dca5120f22e015c19710b2ff04f525720.1711506237.git-series.marmarek@invisiblethingslab.com>
Signed-off-by: Anthony PERARD <anthony@xenproject.org>