virt: move sound device to vmdevices

No code changes. This is part of ongoing effort to
move virtual devices from vm.py to vmdevices.py.

Change-Id: I42b8ae3475373d39c7d6375ccbc23a85437e19d0
Signed-off-by: Martin Polednik <mpolednik@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29265
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>