core: Fixing JSON derialization of VM

Adding another JsonIgnore to JsonVmMixIn.
Apararently @JsonIgnore annotation directly on the getter at Vm class does not work.
This still needs to be investigated, but for now we should not block this fix, as
the exception causes tasks to remain in DB after creating a snapshot.

Change-Id: I1cb86a047d8516fba7c45177bebb83490389e9da
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>