scheduler: Introduce pcpu_schedule_lock

scheduler: Introduce pcpu_schedule_lock

Many places in Xen, particularly schedule.c, grab the per-cpu spinlock
directly, rather than through vcpu_schedule_lock().  Since the lock
pointer may change between the time it's read and the time the lock is
successfully acquired, we need to check after acquiring the lock to
make sure that the pcpu's lock hasn't changed, due to cpu
initialization or cpupool activity.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>