xen/netfront: teardown the device before unregistering it.

Fixes:
[   15.470311] WARNING: at /local/scratch/ianc/devel/kernels/linux/fs/sysfs/file.c:498 sysfs_attr_ns+0x95/0xa0()
[   15.470326] sysfs: kobject eth0 without dirent
[   15.470333] Modules linked in:
[   15.470342] Pid: 12, comm: xenwatch Not tainted 3.4.0-x86_32p-xenU #93
and
[    9.150554] BUG: unable to handle kernel paging request at 2b359000
[    9.150577] IP: [<c1279561>] linkwatch_do_dev+0x81/0xc0
[    9.150592] *pdpt = 000000002c3c9027 *pde = 0000000000000000
[    9.150604] Oops: 0002 [#1] SMP
[    9.150613] Modules linked in:

This is http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=675190



Reported-by: George Shuklin <george.shuklin@gmail.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Tested-by: William Dauchy <wdauchy@gmail.com>
Cc: stable@kernel.org
Cc: 675190@bugs.debian.org
Signed-off-by: David S. Miller <davem@davemloft.net>