xen: add assertion in default_vcpu0_location to protect against broken masks

xen: add assertion in default_vcpu0_location to protect against broken masks

When setting up the cpu sibling/etc masks on ARM I accidentally and
incorrectly omitted a CPU from it's own sibling mask which caused this
function to return an invalid cpu number which caused errors later when we
tried to access per_cpu data for that invalid cpu.

Add an assert to catch this in the future.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>
Committed-by: Ian Campbell <ian.campbell@citrix.com>