Replace expose-event handler with draw handler in GTK3

In GTK3 the 'expose-event' signal has been replaced by a new
'draw' signal. The only difference is that the latter will
pre-create the cairo drawing context & set the clip mask.
Since the drawing code is already structured in a nice way,
we can just wire up the 'gd_draw_event' method to the 'draw'
signal in GTK3

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1361805646-6425-10-git-send-email-berrange@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
