virttest.qemu_vm: Fix deactivate_nic functions.

When send nic_del_cmd by human monitor.send_args_cmd,
the param "convert" means whether we should convert
a command in the format "command params value" to a
proper format that QEMU would accept. Therefore,
its value has to be True rather than False.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>