tests: add missing attribute in LibvirtMock object

testLibvirtconnection.testCallSucceeded raises AttributeError since
LibvirtMock.virEventRunDefaultImpl is missing.

This patch adds the missing attribute.

Change-Id: I4e8f14aafd4561b35b6987a8524f62b91aea50d8
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23374
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>