control: Always explicitly depend on libvirt0

dpkg-shlibdeps will figure out these dependencies exist and
include them in ${shlib:Depends}, but they will look like

  Depends: libvirt0 (>= 7.0.0)

which can sometimes end up not being strict enough.

In order to ensure that all parts of libvirt that are tightly
coupled are always upgraded at the same time, make these
dependencies explicit.