Avoid unnecessary error messages handling udev events

The udev monitor thread "udevEventHandleThread()" will lag the
actual/real view of devices in sysfs as it serially processes udev
monitor events. So for instance if you were to run the following cmd
to create a new veth pair and rename one of the veth endpoints

you might see the following monitor events and real world that looks like

time
| create v0 sysfs entry
wake udevEventHandleThread | create v1 sysfs entry
udev_monitor_receive_device(v1-add) | move v0 sysfs to v2
udevHandleOneDevice(v1) |
udev_monitor_receive_device(v0-add) |
udevHandleOneDevice(v0) | <--- error msgs in virNetDevGetLinkInfo()
udev_monitor_receive_device(v2-move) | as v0 no longer exists
udevHandleOneDevice(v2) |
\/

As you can see the changes in sysfs can take place well before we get
to act on the events in the udevEventHandleThread(), so by the time we
get around to processing the v0 add event, the sysfs entry has been
moved to v2.

To work around this we check if the sysfs entry is valid before
attempting to read it and don't bother trying to read link info if
not. This is safe since we will never read sysfs entries earlier than
it existing, ie. if the entry is not there it has either been removed
in the time since we enumerated the device or something bigger is
busted, in either case, no sysfs entry, no link info. In the case
described above we will eventually get the link info as we work
through the queue of monitor events and get to the 'move' event.

https://bugzilla.redhat.com/show_bug.cgi?id=1557902

Signed-off-by: Mark Asselstine <mark.asselstine@windriver.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>