fix return code type and enum them in ResultCodes

Fix broken migrate and destory VM verbs due to
incompatible return code type of String instead of Integer

  "code" : 0 // and not "0"

also enumerate all result code map in ResultCodes enum
so HashMaps are not created over and onver again, except verbs who
needs to mutable one.

ResultCodes.map() -> returns immutable, unmodifiable map instance
ResultCodes.newMap() -> returns a new, mutable map instance

Change-Id: Id31ea584c36c4809af9c68db04ee7d5d6945a5e7
Signed-off-by: Roy Golan <rgolan@redhat.com>