xen/fb: fix xenfb suspend/resume race.

When migrating guests over a long period we hit this:

<1>BUG: unable to handle kernel paging request at 0000000b819fdb98
<1>IP: [<ffffffff812a588f>] notify_remote_via_irq+0x13/0x34
<4>PGD 94b10067 PUD 0
<0>Oops: 0000 [#1] SMP
.. snip..
Call Trace:
 [<ffffffff812712c9>] xenfb_send_event+0x5c/0x5e
 [<ffffffff8100ea5f>] ? xen_restore_fl_direct_end+0x0/0x1
 [<ffffffff81438d80>] ? _spin_unlock_irqrestore+0x16/0x18
 [<ffffffff812714ee>] xenfb_refresh+0x1b1/0x1d7
 [<ffffffff81270568>] ? sys_imageblit+0x1ac/0x458
 [<ffffffff81271786>] xenfb_imageblit+0x2f/0x34
 [<ffffffff8126a3e5>] soft_cursor+0x1b5/0x1c8
 [<ffffffff8126a137>] bit_cursor+0x4b6/0x4d7
 [<ffffffff8100ea5f>] ? xen_restore_fl_direct_end+0x0/0x1
 [<ffffffff81438d80>] ? _spin_unlock_irqrestore+0x16/0x18
 [<ffffffff81269c81>] ? bit_cursor+0x0/0x4d7
 [<ffffffff812656b7>] fb_flashcursor+0xff/0x111
 [<ffffffff812655b8>] ? fb_flashcursor+0x0/0x111
 [<ffffffff81071812>] worker_thread+0x14d/0x1ed
 [<ffffffff81075a8c>] ? autoremove_wake_function+0x0/0x3d
 [<ffffffff81438d80>] ? _spin_unlock_irqrestore+0x16/0x18
 [<ffffffff810716c5>] ? worker_thread+0x0/0x1ed
 [<ffffffff810756e3>] kthread+0x6e/0x76
 [<ffffffff81012dea>] child_rip+0xa/0x20
 [<ffffffff81011fd1>] ? int_ret_from_sys_call+0x7/0x1b
 [<ffffffff8101275d>] ? retint_restore_args+0x5/0x6
 [<ffffffff81012de0>] ? child_rip+0x0/0x20
Code: 6b ff 0c 8b 87 a4 db 9f 81 66 85 c0 74 08 0f b7 f8 e8 3b ff ff ff c9
c3 55 48 89 e5 48 83 ec 10 0f 1f 44 00 00 89 ff 48 6b ff 0c <8b> 87 a4 db 9f
81 66 85 c0 74 14 48 8d 75 f0 0f b7 c0 bf 04 00
RIP  [<ffffffff812a588f>] notify_remote_via_irq+0x13/0x34
 RSP <ffff8800e7bf7bd0>
CR2: 0000000b819fdb98
---[ end trace 098b4b74827595d0 ]---

The root cause of the panic is the race between the resume and reconnect to the backend.

Clearing the 'update_wanted' flag of xenfb before disconnecting from the
backend fixes this issue.

Signed-off-by: Joe Jin <joe.jin@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Tested-by: Gurudas Pai <gurudas.pai@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Andrew Morton <akpm@linux-foundation.org>