Unlike KVM, we won't use percpu variables because Xen already lays down
statically the shared info structure, that includes the vcpu info pointer
for each cpu.

We could in theory use percpu variables to store pointers to the current cpu
vcpu info, but I ended up giving up this route.  Since our pcpu implementation
have the overhead of computing addresses anyway, we may as well pay the price
and compute it directly from the xen shared info.

One of the things that comes with it, is that we can compute precise timings
using xenclock very early. Since we don't have *that* much to do early, it is
unclear if KVM needs to be improved in this regard (probably not), so this
becomes just a slight bonus.