ui: add a D-Bus display backend

ui: add a D-Bus display backend

The "dbus" display backend exports the QEMU consoles and other
UI-related interfaces over D-Bus.

By default, the connection is established on the session bus, but you
can specify a different bus with the "addr" option.

The backend takes the "org.qemu" service name, while still allowing
further instances to queue on the same name (so you can lookup all the
available instances too). It accepts any number of clients at this
point, although this is expected to evolve with options to restrict
clients, or only accept p2p via fd passing.

The interface is intentionally very close to the internal QEMU API,
and can be introspected or interacted with busctl/dfeet etc:

$ ./qemu-system-x86_64 -name MyVM -display dbus
$ busctl --user introspect org.qemu /org/qemu/Display1/Console_0

org.qemu.Display1.Console           interface -         -               -
.RegisterListener                   method    h         -               -
.SetUIInfo                          method    qqiiuu    -               -
.DeviceAddress                      property  s         "pci/0000/01.0" emits-change
.Head                               property  u         0               emits-change
.Height                             property  u         480             emits-change
.Label                              property  s         "VGA"           emits-change
.Type                               property  s         "Graphic"       emits-change
.Width                              property  u         640             emits-change
[...]

See the interfaces XML source file and Sphinx docs for the generated API
documentations.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>