Bug 1460411 - Add kvm to desktop1604-test image; r=jmaher

Our normal ubuntu 16.04 test image is suitable for hosting an Android x86
emulator with these minor updates: Install kvm and make sure /dev/kvm
rw permissions are open for everyone. Note that /dev/kvm is generally
only visible when running docker with --privileged; its permissions
cannot be modified in the Dockerfile, only at run-time: run-task is the
first opportunity.

taskcluster/docker/recipes/ubuntu1604-test-system-setup.sh

@@ -56,6 +56,7 @@ apt_packages+=('net-tools')
 apt_packages+=('pulseaudio')
 apt_packages+=('pulseaudio-module-bluetooth')
 apt_packages+=('pulseaudio-module-gconf')
+apt_packages+=('qemu-kvm')
 apt_packages+=('rlwrap')
 apt_packages+=('screen')
 apt_packages+=('software-properties-common')

taskcluster/scripts/run-task

@@ -497,6 +497,11 @@ def main(args):
     else:
         uid = gid = gids = None
 
+    if os.path.exists("/dev/kvm"):
+        # Ensure kvm permissions for worker, required for Android x86
+        st = os.stat("/dev/kvm")
+        os.chmod("/dev/kvm", st.st_mode | 0666)
+
     # Validate caches.
     #
     # Taskgraph should pass in a list of paths that are caches via an