vm: drop unused MigrationSourceThreadClass abstraction

Since the libvirtvm.py merger into vm.py (commit ee082b87), we have only
one MigrationSourceThreadClass. Thus, there is no need for this
abstraction.

Change-Id: Ia9fac9aa5b66b68c7447df0c460d91f5f4415b73
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21560
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>