Install qemu from buster-backports

Nova on Victoria requires qemu to bo no less then 4.0.0. To satisfy
this requirement we need to install qemu from backports repo

Change-Id: I6e8fbfb8dec58933f854fd0688eb6030a2110279

tasks/nova_install.yml

@@ -25,6 +25,16 @@
     filter: ansible_local
     gather_subset: "!all"
 
+- name: Set apt package pins
+  include_role:
+    name: apt_package_pinning
+  vars:
+    apt_package_pinning_file_name: "qemu_backports.pref"
+    apt_package_pinning_priority: "1000"
+    apt_pinned_packages: [{ package: 'qemu*', release: 'Debian Backports' }]
+  when:
+    - ansible_distribution | lower == 'debian'
+
 - name: Install distro packages
   package:
     name: "{{ nova_package_list }}"