Merge branch 'libvirt' into 'master'

test: use vagrant libvirt provider as needed

See merge request !56

test/run-test.sh

@@ -28,7 +28,7 @@ clean() {
 start_vagrant() {
     # Ignore errors on update.
     vagrant box update
-    vagrant up \
+    vagrant up ${LIBVIRT:+--provider libvirt} \
         || die "could not start virtual machines"
 }