core: hotfix daotests in VmStaticDAOTest

testGetOrderedVmGuidsForRunMultipleActionsByMigrationSupport which tested db
logic for ordered VmGuids retrieval (with respect to MigrationSupport
attribute) was failing because the number of tested vms was 4, but
MigrationSupport has only 3 possible values. Therefore there were 2 VMs that
were equal with respect to this ordering.

Hot-fixed by restricting the number of test VMs to 3.

Change-Id: I0ea611effabb69a693194a6a0639f2d3c17d08e5
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>