What?
-----

Install the  'libvirt-admin' RPM by default in the 'nova_libvirt'
container: it brings us the useful `virt-admin` tool.

Why?
----

The `virt-admin` tool lets you set the libvirt debug log filters 
dynamically, *without* having to the disruptive process of restarting
the 'libvirtd' process (i.e. restart of the 'nova_libvirt' container).

This is extremely useful for troubleshooting all libvirt/QEMU problems.

How?
----

Here is how to configure libvirt log filters dynamically:

    a. Install the `virt-admin` tool, provided by 'libvirt-admin'
      package:

        $> dnf install libvirt-admin -y

    b. Configure the libvirt log file:

        $> virt-admin daemon-log-outputs "1:file:/var/log/libvirt/libvirtd.log"

    c. Configure the most useful log filters.  (They also let you
       observe the communication between libvirt and QEMU -- e.g. QMP
       commands libvirt sends to QEMU.  They are useful to debug bugs
       involving interactions between libvirt and QEMU):

        $> virt-admin daemon-log-filters \
            "1:libvirt 1:qemu 1:conf 1:security 3:event 3:json 3:file 3:object 1:util"

       Ensure that the filters are actually applied by running
       `virt-admin daemon-log-filters` (with no extra args), which
       should show the log filters that are currently applied.
       
       That's it; no need to restart the libvirt daemon.  The
       /var/log/libvirt/libvirtd.log will now start capturing the debug
       logs with the applied filters.

    d. Now, repeat the test.

    e. Then attach the libvirtd.log (from all relevant hosts) as plain
       text to the bug.

To turn off the dynamic log filters from the command-line:

        $> virt-admin daemon-log-filters ""

Check that the filters are actually removed, by running `virt-admin
daemon-log-filters` (with no extra args), which should show an empty
list.