Debian: update libvirtd init script path

Lately [1], Debian packaging changed to use /etc/default/libvirtd instead of
/etc/default/libvirt-bin (probably to align with systemd, since in Red
Hat it's /etc/sysconfig/libvirtd).

This patch aims to modifify nova::migration::libvirt to build the right
init script path from a param in nova::compute::libvirt
(libvirt_service_name).

[1] http://anonscm.debian.org/cgit/pkg-libvirt/libvirt.git/commit/debian/libvirt-bin.maintscript?id=28dd3618a9a2a8f769d1fa12c28647c4599d9e8e

Change-Id: I7483efe644d301c2646e4063fe7f1aef3e83cb6b
Closes-bug: #1381172