configure.ac: check for libnl-3 in addition to libnl-1

src/Makefile.am: link against libnl when needed

src/util/virnetlink.c:
support libnl3 api.  To minimize impact on code flow, wrap the
differences under the virNetlink* namespace.

Unfortunately libnl3 moves netlink/msg.h to
/usr/include/libnl3/netlink/msg.h, so the LIBNL_CFLAGS need to be added
to a bunch of places where they weren't needed with libnl1.
Signed-off-by: NSerge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: NEric Blake <eblake@redhat.com>