Fix configuring libvirt before installation

When enabling nova::migration::libvirt, it will try to configure
libvirtd.conf before installing libvirt package. This patchset adds
a dependency to make sure libvirt package has been installed when
running nova::migration::libvirt.

Change-Id: I981217c2bbecb276f25fa98d897e1da93d7e02a7
Closes-bug: #1254372
Merge "Fix configuring libvirt before installation"