Default to 127.0.0.1 if /domain/devices/graphics/@listen is missing

libvirt < 0.10.0 doesn't add the default listen address to the
virDomainGetXMLDesc() output.
Ensure AspectBin child emits size-request signal

AspectBin's size-request handler wasn't refreshing the child's size
request.
Pass grab_focus() through AspectBin

When AspectBin is asked to grab focus, pass the request on to the child.
Wrap SpiceClientGtk.Display in AspectBin

SpiceClientGtk.Display added the only-downscale property in spice-gtk
0.14, but Fedora 17 and Debian Wheezy have 0.12.  Stop using the property
and handle this ourselves with AspectBin.