virt: graphdev: add hasSpice helper

add an helper property to display if a Vm has a SPICE display
or not.

Change-Id: I170e439f54a16c2616b57533505ad65e8c06490f
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26892
Reviewed-by: Dan Kenigsberg <danken@redhat.com>