core: Fix CreateAllSnapshotsFromVmCommandTest

Fix test-break caused by commit
665c1c9e243eaa6ff16cbcdc88828b3645009f8a.

The aforementioned patch moves the verification if a VM is (not)
external from VmHandler to the VM entity itself, and was thus no
longer mocked by the test, causing the failure.

This patch adds such mocking, to allow the test to pass.

Change-Id: I298d3bd0c05f3175371806dec60c7dccb798d165
Signed-off-by: Allon Mureinik <amureini@redhat.com>