qemu: add tests for standalone VNC graphics

qemu: add tests for standalone VNC graphics

Add xml2argv tests verifying the standalone VNC feature:

- graphics-vnc-standalone: when both <graphics type='dbus'/> and
  <graphics type='vnc'/> are present, the -vnc QEMU argument is
  omitted since qemu-vnc handles VNC externally via D-Bus.

- graphics-vnc-standalone-socket: same behavior with a Unix socket
  listen address.

- graphics-vnc-standalone-p2p: when dbus is p2p mode, standalone
  VNC is NOT triggered and the built-in -vnc argument is preserved.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>