Until now, the nl_pid of the source address of every message sent by
virNetlinkCommand has been set to the value of getpid(). Most of the
time this doesn't matter, and in the one case where it does
(communication with lldpad), it previously was the proper thing to do,
because the netlink event service (which listens on a netlink socket
for unsolicited messages from lldpad) coincidentally always happened
to bind with a local nl_pid == getpid().

With the fix for:

  https://bugzilla.redhat.com/show_bug.cgi?id=816465

that particular nl_pid is now effectively a reserved value, so the
netlink event service will always bind to something else
(coincidentally "getpid() + (1 << 22)", but it really could be
anything). The result is that communication between lldpad and
libvirtd is broken (lldpad gets a "disconnected" error when it tries
to send a directed message).

The solution to this problem caused by a solution, is to query the
netlink event service's nlhandle for its "local_port", and send that
as the source nl_pid (but only when sending to lldpad, of course - in
other cases we maintain the old behavior of sending getpid()).

There are two cases where a message is being directed at lldpad - one
in virNetDevLinkDump, and one in virNetDevVPortProfileOpSetLink.

The case of virNetDevVPortProfileOpSetLink is simplest to explain -
only if !nltarget_kernel, i.e. the message isn't targetted for the
kernel, is the dst_pid set (by calling
virNetDevVPortProfileGetLldpadPid()), so only in that case do we call
virNetlinkEventServiceLocalPid() to set src_pid.

For virNetDevLinkDump, it's a bit more complicated. The call to
virNetDevVPortProfileGetLldpadPid() was effectively up one level (in
virNetDevVPortProfileOpCommon), although obscured by an unnecessary
passing of a function pointer. This patch removes the function
pointer, and calls virNetDevVPortProfileGetLldpadPid() directly in
virNetDevVPortProfileOpCommon - if it's doing this, it knows that it
should also call virNetlinkEventServiceLocalPid() to set src_pid too;
then it just passes src_pid and dst_pid down to
virNetDevLinkDump. Since (src_pid == 0 && dst_pid == 0) implies that
the kernel is the destination, there is no longer any need to send
nltarget_kernel as an arg to virNetDevLinkDump, so it's been removed.

The disparity between src_pid being int and dst_pid being uint32_t may
be a bit disconcerting to some, but I didn't want to complicate
virNetlinkEventServiceLocalPid() by having status returned separately
from the value.