core: Ignoring specParams when serialization

SpecParams isn't needed in serialization and fails the desialization due
to abstract type usage (java.util.Map)

JsonIgnore on a getter method simply doesn't work at current Jackson impl
unless used in a MixIn.

Added a new Mixin for VmPayload
Removed @JsonIgnore from VMPayload class

Bug-Url: https://bugzilla.redhat.com/985008
Change-Id: I33a6e38a83a715aa359497e0089c16b610d1234a
Signed-off-by: Roy Golan <rgolan@redhat.com>