xen/x86: Initialize per_cpu(xen_vcpu, 0) a little earlier

xen_cpuhp_setup() calls mutex_lock() which, when CONFIG_DEBUG_MUTEXES
is defined, ends up calling xen_save_fl(). That routine expects
per_cpu(xen_vcpu, 0) to be already initialized.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>