add testcase for spawn method
add testcase for spawn with 'boot from volume'
tests: add reference to Server to fakeprlsdkapi.Vm

It's needed for reg and destroy operations.
tests: always add sdk vms to vm list

It's better to use Server.test_add_vm instead of
creating Vm class by hand, because Vm's must be added
to Vm list in Server instance.
tests: improve _prep_instance* methods

Add **kwargs to these methods, so you will be able to generate
instance objects with any fields.
tests: add testcases for Driver.destroy method
tests: add testcases for instance state switching