virtme-run: Add Xen support

This can boot a Xen hypervisor with a selected kernel loaded as dom0.
It doesn't work, though: it hangs when trying to mount virtfs.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Merge Xen support

This will hang at boot unless you have a fixed virtio driver in the
guest.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>