core: fix JSON serialization issues in VmPayload

Added JsonIgnore to getSpecParams as its not a field
Changed "files" member type to HashMap as it will fail in
deserialization

See also Bug 996005

Change-Id: Ief9e04e12a05b39cb5d2bceea5a23fbb05d01b09
Signed-off-by: Roy Golan <rgolan@redhat.com>
Bug-Url: https://bugzilla.redhat.com/985008