libvirt.tests: a bunch of amendments to vfio tests.

* Support to provide gateway for network device.
* Change the way to remove boot elements in os.
  Because when there is not boot_order for vm, libvirt
  will add boot element to 'hd' automatically.
  We have to remove it after adding boot_order for devices.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>