vm: return displayIp as a string in default case

Previously, default value of displayInfo.displayIp was number 0. This patch
changes it to string '0' so that it conforms to the schema.

Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Change-Id: If211e80d731de27dd5d5154c32dd63bef5c2be2a
Reviewed-on: http://gerrit.ovirt.org/34675
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>