Debian: use QEMU 5.0 from buster-backports

This will allow us to make sure of all updates.

'qemu-kvm' got dropped in Debian

Change-Id: If09a8188030baa284dd353b1c3f830d78c5091b7

docker/base/apt_preferences.debian

@@ -41,3 +41,8 @@ Pin-Priority: 700
 Package: openvswitch*
 Pin: release n=buster
 Pin-Priority: 700
+
+# we want newer QEMU
+Package: qemu*
+Pin: version 1:5.0*
+Pin-Priority: 700

docker/nova/nova-libvirt/Dockerfile.j2

@@ -71,8 +71,13 @@ LABEL maintainer="{{ maintainer }}" name="{{ image_name }}" build-date="{{ build
     {% elif base_arch == "x86_64" %}
         {% set nova_libvirt_packages = nova_libvirt_packages + [
             'ovmf',
-            'qemu-kvm'
         ] %}
+        # NOTE(hrw): Debian merged 'qemu-kvm' into 'qemu-system-ARCH' in 5.0-9
+        {% if base_distro == 'ubuntu' %}
+            {% set nova_libvirt_packages = nova_libvirt_packages + [
+                'qemu-kvm'
+            ] %}
+        {% endif %}
     {% endif %}
 
     {% if base_arch in ['aarch64', 'x86_64'] %}