One of the top questions by libvirt users is how to create a host
bridge device so that guests can be directly on the physical
network. There are several example documents that explain how to do
this manually, but following them often results in confusion and
failure. virt-manager does a good job of creating a bridge based on an
existing network device, but not everyone wants to use virt-manager.

This patch adds a new command, iface-bridge that makes it just about
as simple as possible to create a new bridge device based on an
existing ethernet/vlan/bond device (including associating IP
configuration with the bridge rather than the now-attached device),
and start that new bridge up ready for action, eg:

    virsh iface-bridge eth0 br0

For symmetry's sake, it also adds a command to remove a device from a
bridge, restoring the IP config to the now-unattached device:

    virsh iface-unbridge br0

(I had a short debate about whether to do "iface-unbridge eth0"
instead, but that would involve searching through all bridge devices
for the one that contained eth0, which seems like a bit too much
trouble).

NOTE: These two commands require that the netcf library be available
on the host. Hopefully this will provide some extra incentive for
people using suse, debian, ubuntu, and other similar systems to polish
up (and push downstream) the ports to those distros recently pushed to
the upstream netcf repo by Dan Berrange. Anyone interested in helping
with that effort in any way should join the netcf-devel mailing list
(subscription info at
https://fedorahosted.org/mailman/listinfo/netcf-devel)

During creation of the bridge, it's possible to specify whether or not
the STP protocol should be started up on the bridge and, if so, how
many seconds the bridge should squelch traffic from newly added
devices while learning new topology (defaults are stp='on' and
delay='0', which seems to usually work best for bridges used in the
context of libvirt guests).

There is also an option to not immediately start the bridge (and a
similar option to not immediately start the un-attached device after
destroying the bridge. Default is to start the new device, because in
the case of iface-unbridge not starting is strongly discouraged as it
will leave the system with no network connectivity on that interface
(because it's necessary to destroy/undefine the bridge device before
the unattached device can be defined), and it seemed better to make
the option for iface-bridge behave consistently.

NOTE TO THOSE TRYING THESE COMMANDS FOR THE FIRST TIME: to guard
against any "unexpected" change to configuration, it is advisable to
issue an "virsh iface-begin" command before starting any interface
config changes, and "virsh iface-commit" only after you've verified
that everything is working as you expect. If something goes wrong,
you can always run "virsh iface-rollback" or reboot the system (which
should automatically do iface-rollback).

Aside from adding the code for these two functions, and the two
entries into the command table, the only other change to virsh.c was
to add the option name to vshCommandOptInterfaceBy(), because the
iface-unbridge command names its interface option as "bridge".

virsh.pod has also been updated with short descriptions of these two
new commands.