hack: switch all guests to PV using an external hardcoded kernel

This works around the bugs:
1. HVM secondary consoles don't work
2. my blkfront doesn't like qemu qdisk backends

Signed-off-by: David Scott <dave.scott@citrix.com>
xen: the choice of PV disk model depends on the hypervisorType

If KVM: use VIRTIO
If Xen: use XEN

Signed-off-by: David Scott <dave.scott@citrix.com>