[libvirt] [PATCH 3/5] macvtap support for libvirt -- qemu support

This part adds support for qemu making a macvtap tap device available via file descriptor passed to qemu command line. This also attempts to tear down the macvtap device when a VM terminates. This includes support for attachment and detachment to/from running VM. Signed-off-by: Stefan Berger <stefanb@us.ibm.com>