[libvirt] ESX [08/12]: 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()