locking/qspinlock,x86: Clarify virt_spin_lock_key

Add a few comments to clarify how this is supposed to work.

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Juergen Gross <jgross@suse.com>