tests: fix FakeGuestAgent field

fix guestIPs, which is supposed to be
"A space separated string of assigned IPv4 addresses"
and not a list.

Change-Id: If4d9822356b05a87a75ba4a0b097265b6760e0e4
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28801
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>