Add esxVI_Occurence enum to describe expected occurence of items
* src/esx/esx_driver.c: update the use of esxVI_LookupVirtualMachineByUuid()
* src/esx/esx_vi.c: add an esxVI_Occurence parameter to
  esxVI_LookupVirtualMachineByUuid() and take care if esxVI_FindByUuid()
  can't find anything for a given uuid
* src/esx/esx_vi.h: add esxVI_Occurence enum
* src/esx/esx_vi_methods.c: expect null or more items to be returned
  from esxVI_FindByUuid()