qemu: don't kill qemu process on restart if networkNotify fails

qemu: don't kill qemu process on restart if networkNotify fails

Nothing that could happen during networkNotifyActualDevice() could
justify unceremoniously killing the qemu process, but that's what we
were doing.

In particular, new code added in commit 85bcc022 (first appearred in
libvirt-3.2.0) attempts to reattach tap devices to their assigned
bridge devices when libvirtd restarts (to make it easier to recover
from a restart of a libvirt network). But if the network has been
stopped and *not* restarted, the bridge device won't exist and
networkNotifyActualDevice() will fail.

This patch changes networkNotifyActualDevice() and
qemuProcessNotifyNets() to return void, so that qemuProcessReconnect()
will soldier on regardless of what happens (any errors will still be
logged though).

Partially resolves: https://bugzilla.redhat.com/1442700