This resolves:

   https://bugzilla.redhat.com/show_bug.cgi?id=881480

These three functions:

  virDomainNetGetActualBridgeName
  virDomainNetGetActualDirectDev
  virDomainNetGetActualDirectMode

return attributes that are in a union whose contents are interpreted
differently depending on the actual->type and so they should only
return non-0 when actual->type is 'bridge' (in the first case) or
'direct' (in the other two cases, but I had neglected to do that, so
...DirectDev() was returning bridge.brname (which happens to share the
same spot in the union with direct.linkdev) if actual->type was
'bridge', and ...BridgeName was returning direct.linkdev when
actual->type was 'direct'.

How does this involve Bug 881480 (which was about the inability to
switch between two networks that both have "<forward mode='bridge'/>
<bridge name='xxx'/>"? Whenever the return value of
virDomainNetGetActualDirectDev() for the new and old network
definitions doesn't match, qemuDomainChangeNet() requires a "complete
reconnect" of the device, which qemu currently doesn't
support. ...DirectDev() *should* have been returning NULL for old and
new, but was instead returning the old and new bridge names, which
differ.

(The other two functions weren't causing any behavioral problems in
virDomainChangeNet(), but their problem and fix was identical, so I
included them in this same patch).
(cherry picked from commit 3738cf41)