Specify --doc-main-package for dh_installdocs
Specify --doc-main-package for dh_installdocs

Starting with compat 11, debhelper tries to detect this
itself, but unfortunately it's getting it wrong for us and
picking libvirt-dev instead of libvirt-doc.

Fixes doc-base-file-references-missing-file Lintian error.

agx/libvirt-debian,agx/libvirt-debian,agx/libvirt-debian,agx/libvirt-debian,agx/libvirt-debian