core: temporary fix for vm deserialization

The introduction of graphics-info to VM broke the deserialization of VMs
using json since there is a 'getter' named 'getGraphicsInfos' without a
corresponding setter. This patch makes a temporary change in order to
overcome this problem by adding JsonIgnore annotation on the
getGraphicsInfos method, until a proper solution will be made.

Change-Id: I365000b9dd8e282af0a43932963baf29e6cad7f0
Signed-off-by: Arik Hadas <ahadas@redhat.com>