qemu.tests: Add create_macvtap_device test case and script.

Signed-off-by: Feng Yang <fyang@redhat.com>
qemu.tests.create_macvtap_device: Do not check ping status after Creating a passthru mode macvtap device.

Signed-off-by: Feng Yang <fyang@redhat.com>
qemu.tests: Update create_macvtap_device to cover more case.

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #124 from FengYang/create_macvtap

Add create_macvtap_device test