libvirt: remove unused 'get_disks' method

The 'get_disks' method was a virt driver public API used by
the old compute monitor class. This class was deleted in

  commit 07646e85841a4f7c81e80254ac63715bece2aadd
  Author: Brian Waldon <brian.waldon@rackspace.com>
  Date:   Tue Aug 2 10:09:58 2011 -0400

      removing compute monitor

Leaving no in-tree user of the 'get_disks' method. This
method is not declared in the ComputeDriver base class
nor implemented by any virt driver besides libvirt, so
can be safely deleted.

Partial-bug: #1333219
Change-Id: I4d3f53499d088a82e471ab68feef1ae50d3f8bdc
libvirt: add '_' prefix to all get_guest_*_config methods

The get_guest_*_config methods are all internal helpers so should
have a '_' prefix to indicate they are not part of public virt
driver API.

Related-bug: #1333219
Change-Id: I76108c4992144cc7c3273bfd753aa76f7a821a56
Move the error check for "brctl addif"

The git commit 873aad92944f8840e772d65eda4b3320d65a9ce7[1] has moved
"brctl addif" command, but the commit did not move its error check.

Now the error check sees the result of "ip addr show dev <interface>
scope global" command wrongly, and the command outputs "Dump was
interrupted and may be inconsistent" sometimes because of a kernel change.
As the result, gate failures happen sometimes.

This patch moves the error check to right place.

[1]: https://github.com/openstack/nova/commit/873aad92944f8840e772d65eda4b3320d

Change-Id: I92ee96e3aa34f7cd154a8ce4b33852b96cabe8c9
Closes-Bug: #1334142
Merge "Move the error check for "brctl addif""
Merge "libvirt: remove unused 'get_disks' method"
Merge "libvirt: add '_' prefix to all get_guest_*_config methods"