core: fix coverity issue

Fix CID 1223222:  Se: Incorrect definition of Serializable class
(FB.SE_BAD_FIELD)

The MergeStatusReturnValue member of RemoveSnapshotSingleDiskParameters
did not implement Serializable.

Change-Id: I7363ae80b3f6c6bdcf1f5dbb615d6021cce23872
Signed-off-by: Greg Padgett <gpadgett@redhat.com>