core: Use FixturesTool in VmDAOTest

Removed local constants and hardcoded values and used FixturesTools
constants instead where those existed. Where they didn't, they were
added.

The fixtures.xml file was also slightly fixed to avoid naming confusion
with non-unique names.

Change-Id: Ia3afb7afa9f508ea61ea3bc6502c80d0812d18fa
Signed-off-by: Allon Mureinik <amureini@redhat.com>