autopkg tests: Use isolation-machine

This avoids running under debian ci since libvirt-lxc in lxc
doesn't work there.

Closes: #947006

debian/tests/control

@@ -8,7 +8,7 @@ Restrictions: allow-stderr, isolation-container
 
 Tests: smoke-lxc
 Depends: libvirt-daemon-system, libvirt-daemon-driver-lxc, libvirt-clients, libxml2-utils
-Restrictions: allow-stderr, needs-root
+Restrictions: allow-stderr, needs-root, isolation-machine
 
 Tests: build-test
 Depends: libvirt-dev, build-essential, pkg-config