Bug 4881 - failure in netvirt.AbstractEventTest

Bug 4881 - failure in netvirt.AbstractEventTest

build fails with the following failure.
transaction id of the event can't be assumed to be 1 because other
tests may have created (subclass of) abstract event before running
this test. This patch to get transaction id from the event and use it
instead of hard-coded 1.

> Tests run: 1, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 0.029 sec <<< FAILURE! - in org.opendaylight.ovsdb.openstack.netvirt.AbstractEventTest
> testAbstractEvent(org.opendaylight.ovsdb.openstack.netvirt.AbstractEventTest)  Time elapsed: 0.017 sec  <<< FAILURE!
> org.junit.ComparisonFailure: Error, toString() did not return the correct value expected:<...vent [transactionId=[1] handlerType=SOUTHBO...> but was:<...vent [transactionId=[5] handlerType=SOUTHBO...>
>         at org.junit.Assert.assertEquals(Assert.java:115)
>         at org.opendaylight.ovsdb.openstack.netvirt.AbstractEventTest.testAbstractEvent(AbstractEventTest.java:59)

Change-Id: I1f351b92a684e8ece17246d8f0011cbffe95409f
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>