Rename the src/util/netlink files to src/util/virnetlink to
better fit the naming scheme. Also rename nlComm to virNetlinkCommand.
Signed-off-by: ND. Herrendoerfer <d.herrendoerfer@herrendoerfer.name>

This patch fixes the compilation of netlink.c and interface.c on those
systems missing either libnl or that have an older linux/if_link.h
include file not supporting macvtap or VF_PORTS.

WITH_MACVTAP is '1' if newer include files were detected, '0' otherwise.

IFLA_PORT_MAX is defined in linux/if_link.h if yet more functionality is
supported.

In a first cleanup step, make nlComm from macvtap.c commonly available
for other code to use. Since nlComm uses Linux-specific structures as
parameters it's prototype is only visible on Linux.