[libvirt] [PATCH v3] AppArmor: add rules needed with additional mediation features brought by Linux 4.14.

Changes since v2: - made signal rules broader, as suggested by Jamie Strandboge <jamie@canonical.com> and indeed my tests confirm v2 was too strict; - allowed libvirtd "ptrace (read)" on libvirt-* guests, as suggested by Jamie Strandboge <jamie@canonical.com> - added fine-grained mount rules written by openSUSE's Christian Boltz