qemu: Add macvtap support

This patch introduces using macvtap nic in test, when using
macvtap nic better have a switch support macvtap, if not,
need have a host with two nic devices when using this patch.

if you want to use macvtap in your test, just configure
"nettype = macvtap" in your .cfg file, or provide
--nettype macvtap as a flag for the virt test runner.

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