qemu: prevent unnecessarily failing live interface update

qemu: prevent unnecessarily failing live interface update

Attempts to use update-device to modify just the link state of a guest
interface were failing due to a supposed attempt to modify something
in the interface that can't be modified live (even though the only
thing that was changing was the link state, which *can* be modified
live).

It turned out that this failure happened because the guest interface
in question was type='network', and the network in question was a
'direct' network that provides each guest interface with one device
from a pool of network devices. As a part of qemuDomainChangeNet() we
would always allocate a new port from the network driver for the
updated interface definition (by way of calling
virDomainNetAllocateActualDevice(newdev)), and this new port (ie the
ActualNetDef in newdev) would of course be allocated a new host device
from the pool (which would of course be different from the one
currently in use by the guest interface (in olddev)). Because direct
interfaces don't support changing the host device in a live update,
this would cause the update to fail.

The solution to this is to realize that as long as the interface
doesn't get switched to a different network as a part of the update,
the network port information (ie the ActualNetDef) will not change as
a part of updating the guest interface itself. So for sake of
comparison we can just point the newdev at the ActualNetDef of olddev,
and then clear out one or the other when we're done (to avoid a double
free or, more likely, attempt to reference freed memory).

(If, on the other hand, the name of the network has changed, or if the
interface type has changed to type='network' from something else, then
we *do* need to allocate a new port (actual device) from the network
driver (as we used to do in all cases when the new type was
'network'), and also indicate that we'll need to replace olddev in the
domain with newdev (because either of these changes is major enough
that we shouldn't just try to fix up olddev)

Partially-Resolves: https://issues.redhat.com/browse/RHEL-7036
Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>