Add a simple qemu guest agent package

We don't actually build it here, we just use the alpine package, which is
already included in the linuxkit/alpine base.

Can be instantiated with a service stanza such as:
  - name: qemu-ga
    image: "linuxkit/qemu-ga:e5fbcf55926f6e1a96b3e49a392e547e8be1022c"
    binds:
      - /dev/vport0p1:/dev/vport0p1

The only functionality I've tested is to report IP addresses to the host (e.g.
via `virsh domifaddr`).

The image is configured to use `/dev/vport0p1` by default. If your VM image is
using a different port number then you can either incorporate the rename into
the bind ("/dev/vport1p1:/dev/vport0p1") or override the command with:
    command: ["/usr/bin/qemu-ga", "-p", "/dev/vport1p1"]

The mdev tool appears to not create the symlinks which udev would provide
`/dev/virtio-ports/org.qemu.guest_agent.0` hence the need to hardcode the
potentially unstable `/dev/vportNp1`, `N` seems to depend on the number and
order of virtio devices in use. I don't know if it is possible to get mdev to
create these links. For reference the udev rule is:

    SUBSYSTEM=="virtio-ports", KERNEL=="vport*", ATTR{name}=="?*", SYMLINK+="virtio-ports/$attr{name}"

See https://wiki.libvirt.org/page/Qemu_guest_agent for more info on the guest
agent.

A suitable stanza is added to `projects/swarmd/swarmd.yml` since that is where
I am using it.

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

projects/swarmd/swarmd.yml

@@ -20,6 +20,10 @@ onboot:
   - name: metadata
     image: "linuxkit/metadata:b053fd6a945829bb626bb3546cad69aae0ff7803"
 services:
+  - name: qemu-ga
+    image: "linuxkit/qemu-ga:e5fbcf55926f6e1a96b3e49a392e547e8be1022c"
+    binds:
+      - /dev/vport0p1:/dev/vport0p1
   - name: rngd
     image: "linuxkit/rngd:b67c3151a52b05db50e6207b40876900f2208d14"
   - name: ntpd