tests: backport assertAlmostEqual

vm migration tests depend on the 'delta'
keyword argument of the assertAlmostEqual method.

This patch adds the backport of this method,
thus unbreaking the tests on python 2.6.

Change-Id: I5fecc424fec13b01b4a9cdb81f576edb9887c3d3
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35156
Tested-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>