Remove unused methods

 o nova/cells/messaging.py
   - def _at_max_hop_count(self, do_raise=True):

     This method was added in the following commit.

        commit f9a868e86ce11f786538547c301b805bd68a1697
        Author: Chris Behrens <cbehrens@codestud.com>
        Date:   Fri Apr 13 05:54:48 2012 +0000

            Cells: Add the main code.

     But there is no evidence that was used.

 o nova/virt/disk/mount/nbd.py
   - def _read_pid_file(self, pidfile):

     This method was added in the following commit.

        commit e4377fdb0ef2087a36d2b1fbee96543f735040de
        Author: Michael Still <michael.still@canonical.com>
        Date:   Wed Dec 12 17:13:53 2012 +1100

            Stop nbd leaks, remove pid race.

     But there is no evidence that was used.

 o nova/virt/hyperv/imagecache.py
   - def _validate_vhd_image(self, vhd_path):

     This method was removed in the following commit.

        commit 689e9e3e3d48928fe950b26745a7df6fee2f0718
        Author: Alessandro Pilotti <ap@pilotti.it>
        Date:   Mon Mar 4 11:34:42 2013 +0200

            Fixes disk size issue during image boot on Hyper-V

 o nova/virt/hyperv/vmutils.py
   - def _clone_wmi_obj(self, wmi_class, wmi_obj):

     This method was removed in the following commit.

        commit d1a2e108695690b66376d406d4a5038c09c84335
        Author: Alessandro Pilotti <apilotti@cloudbasesolutions.com>
        Date:   Tue Jul 30 03:58:11 2013 +0300

            Improves Hyper-V vmutils module for subclassing

 o nova/virt/libvirt/firewall.py
   - def _define_filters(self, filter_name, filter_children):

     This method was removed in the following commit.

        commit b4fae4821fd2d4f31c0c03bdf2e65c984f42e01d
        Author: Yaguang Tang <heut2008@gmail.com>
        Date:   Tue Feb 14 10:43:39 2012 +0800

            remove unused nwfilter methods and tests.

 o nova/virt/xenapi/volume_utils.py
   - def _get_volume_id(path_or_id):

     This method was removed in the following commit.

        commit 503d5729547f135951d75d5237b398fe1700aaf2
        Author: Armando Migliaccio <amigliaccio@internap.com>
        Date:   Fri Dec 7 21:06:43 2012 +0000

           Fixes KeyError: 'sr_uuid' when booting from volume on xenapi

Change-Id: I216f7c4ff3546c41e1fb0813a71a6c7ff9498584
Merge "Remove unused methods"