util: netlink function to delete any network device

util: netlink function to delete any network device

libvirt has always used the netlink RTM_DELLINK message to delete
macvtap/macvlan devices, but it can actually be used to delete other
types of network devices, such as bonds and bridges. This patch makes
virNetDevMacVLanDelete() available as a generic function so it can
intelligibly be called to delete these other types of interfaces.