Description of problem:

When creating a guest with libvirt for KVM real-time, we always
drop unneeded devices from the domain's XML leading to a very
minimal hardware support for a guest.

We do this for two reasons:

 1. We just don't know at this point which drivers in the guest
    may be real-time unfriendly, which could cause latency
    spikes during real-time operation (eg. a driver that creates
    a polling thread on all CPUs)

 2. We know that some hardware does cause problems with real-time
    in bare-metal (eg. legacy USB support), but we don't know
    whether such hardware will also be problematic in guests

Maybe we'll have some kind of device validation in the future,
but for now we're playing safe and dropping all the unnecessary
devices from the XML.

The list of dropped devices includes:

 - All USB support
 - All SPICE support and QXL hardware
 - virtio serial and guest agent sockets
 - Sound card

Needless to say, but we keep some virtio devices like the disk
and the NIC. Having the disk or the NIC as emulated devices
_might_ be OK as long as the real-time thread don't do I/O to
them.

If OpenStack, or another virt stack component, has a specific
request for a missing device we could try to validate it and
include it.


Version-Release number of selected component (if applicable):

OSP8

How reproducible:

Always

Steps to Reproduce:
1. Create a realtime KVM guest, with "unneeded" devices listed above;
2. Observe the presence of these "unneeded" devices in the domain XML.


Actual results:
Inappropriate devices (such as emulated devices) may be included
in the realtime guest. These inappropriate devices may lead to 
undesirable latency spikes.

Expected results:
"Unneeded devices" (or inappropriate devices for realtime) should be 
removed from the XML.

Additional info: