xen/smp: Fix leakage of timer interrupt line for every CPU online/offline.

In the PVHVM path when we do CPU online/offline path we would
leak the timer%d IRQ line everytime we do a offline event. The
online path (xen_hvm_setup_cpu_clockevents via
x86_cpuinit.setup_percpu_clockev) would allocate a new interrupt
line for the timer%d.

But we would still use the old interrupt line leading to:

kernel BUG at /home/konrad/ssd/konrad/linux/kernel/hrtimer.c:1261!
invalid opcode: 0000 [#1] SMP
RIP: 0010:[<ffffffff810b9e21>]  [<ffffffff810b9e21>] hrtimer_interrupt+0x261/0x270
.. snip..
 <IRQ>
 [<ffffffff810445ef>] xen_timer_interrupt+0x2f/0x1b0
 [<ffffffff81104825>] ? stop_machine_cpu_stop+0xb5/0xf0
 [<ffffffff8111434c>] handle_irq_event_percpu+0x7c/0x240
 [<ffffffff811175b9>] handle_percpu_irq+0x49/0x70
 [<ffffffff813a74a3>] __xen_evtchn_do_upcall+0x1c3/0x2f0
 [<ffffffff813a760a>] xen_evtchn_do_upcall+0x2a/0x40
 [<ffffffff8167c26d>] xen_hvm_callback_vector+0x6d/0x80
 <EOI>
 [<ffffffff81666d01>] ? start_secondary+0x193/0x1a8
 [<ffffffff81666cfd>] ? start_secondary+0x18f/0x1a8

There is also the oddity (timer1) in the /proc/interrupts after
offlining CPU1:

  64:       1121          0  xen-percpu-virq      timer0
  78:          0          0  xen-percpu-virq      timer1
  84:          0       2483  xen-percpu-virq      timer2

This patch fixes it.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: stable@vger.kernel.org