xen: move vcpu_info to common, leave hook for setup

vcpu_info is crucial for the Xen event channel core. Since both the
data and setup steps are identical between architectures, move them to
the common file. Since there is no cross-architecture method to call
a function on every processor during bring-up, simply leave the setup
function.

The number of vcpu_info structures available on the shared information
page varies by architecture. Instead of hard-coding the count use
nitems(). Add a warning message for this being used.

Switch to XEN_VCPUID() and use Xen's typedefs.

panic() on failure since >32 processors is no longer unusual.

royger: Specify 64-byte alignment for vcpu_info to try to defend
against vcpu_info crossing a page boundary. Add detection for this
limit.

Reviewed by: royger