x86/hvm: fix use-after-free introduced by c/s 428607a

x86/hvm: fix use-after-free introduced by c/s 428607a

c/s 428607a "x86: shrink 'struct domain', was already PAGE_SIZE" introduced a
use-after-free error during domain destruction, because of the order in which
timers are torn down.

  (XEN) Xen call trace:
  (XEN)    [<ffff82d08013344e>] spinlock.c#check_lock+0x1e/0x40
  (XEN)    [<ffff82d08013349b>] _spin_lock+0x11/0x52
  (XEN)    [<ffff82d0801e8076>] vpt.c#pt_lock+0x24/0x40
  (XEN)    [<ffff82d0801e88f4>] destroy_periodic_time+0x18/0x81
  (XEN)    [<ffff82d0801e1089>] rtc_deinit+0x53/0x78
  (XEN)    [<ffff82d0801d1e5a>] hvm_domain_destroy+0x52/0x69
  (XEN)    [<ffff82d08016a758>] arch_domain_destroy+0x1a/0x98
  (XEN)    [<ffff82d080107cd5>] domain.c#complete_domain_destroy+0x6f/0x182
  (XEN)    [<ffff82d080126a19>] rcupdate.c#rcu_process_callbacks+0x144/0x1a6
  (XEN)    [<ffff82d080132c52>] softirq.c#__do_softirq+0x82/0x8d
  (XEN)    [<ffff82d080132caa>] do_softirq+0x13/0x15
  (XEN)    [<ffff82d080248ae1>] entry.o#process_softirqs+0x21/0x30
  (XEN)
  (XEN)
  (XEN) ****************************************
  (XEN) Panic on CPU 3:
  (XEN) GENERAL PROTECTION FAULT
  (XEN) [error_code=0000]
  (XEN) ****************************************

Defer the freeing of d->arch.hvm_domain.pl_time until all timers have been
destroyed.

For safety, NULL out the pointers after freeing them, in an attempt to make
mistakes more obvious in the future.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>