qemu.test.vlan: Fix logic error.

Signed-off-by: Feng Yang <fyang@redhat.com>

On 12/29/2014 11:53 AM, Xu Tian wrote:
>
> In generic/tests/vlan.py
> <https://github.com/autotest/tp-qemu/pull/207#discussion-diff-22302579>:
>
> > @@ -76,10 +76,8 @@ def rem_vlan(session, v_id, iface="eth0", cmd_type="ip"):
> >              err_msg = "Unexpected vlan operation command: %s, " % cmd_type
> >              err_msg += "only support 'ip' and 'vconfig' now"
> >              raise error.TestError(err_msg)
> > -
> > -        send_cmd = "[ -e /proc/net/vlan/%s ] && %s" % (v_iface, rem_vlan_cmd)
>
> Why drop command that used to check vlan exist?
>
This command is not stable and always return wrong status.

Also this command is not necessary for the remove function.

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> <https://github.com/autotest/tp-qemu/pull/207/files#r22302579>.
>