virt: migration: move threads in a separate module

This patch moves the migration control threads
in a separate module.

No functional/behavioural changes.

Change-Id: Ia69c7448b66417c7bba8ae3d301e7d777ca88067
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25970
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>