Refactor libvirt/linux_net integration to be portable to FreeBSD

For quite some time already libvirt's Qemu driver works on FreeBSD. And the goal is to make nova-compute run with libvirt/qemu driver on FreeBSD host. Additionally, the ultimate goal would be to get libvirt/bhyve support to work on FreeBSD as well, this definitely should be a separate blueprint, however, the preparation work for getting nova-compute run on FreeBSD, either with libvirt/qemu or libvirt/bhyve, is the same, so I mention it to make the long-term goals clear.

Currently implementation is that nova.virt.libvirt.vif directly imports and uses nova.network.linux_net. In order to make it flexible, the following refactoring is proposed:

 - implement nova.network.freebsd_net with a minimal required feature set for libvirt.vif
 - refactor libvirt.vif to allow configure network driver to use instead of hardcoding 'linux_net'