migraton/libvirt: fix dependency cycle

In the last commit, I introduced a dependency cyle:
(File_line[/etc/sysconfig/libvirtd libvirtd args] => Service[libvirt] =>
Class[Nova::Compute::Libvirt] => File_line[/etc/sysconfig/libvirtd
libvirtd args])

The "require" is useless, since the migration class is called by
nova::compute::libvirt, let's delete it.

Change-Id: I97aeb9dcc11bb867e020826ad9ed5d7d0e04a8e8