CA-198956: VM.set_snapshot_schedule throws exception "Object deleted:VMSS" in xapi
CA-198956: VM.set_snapshot_schedule throws exception "Object deleted:VMSS" in xapi

XAPI expects "OpaqueRef:NULL" in the value field to unassign a VM from
a particular policy, however XC is sending an empty string "". As a fix
the empty string is replaced with Helper.NullOpaqueRef.

Signed-off-by: Sharath Babu <5d7122acc94bd194b321162c746602b350decf46@citrix.com>

huizh/xenadmin,stephen-turner/xenadmin,letsboogey/xenadmin,GaborApatiNagy/xenadmin,ushamandya/xenadmin,geosharath/xenadmin,xenserver/xenadmin,GaborApatiNagy/xenadmin,huizh/xenadmin,jijiang/xenadmin,minli1/xenadmin,letsboogey/xenadmin,GaborApatiNagy/xenadmin,geosharath/xenadmin,MihaelaStoica/xenadmin,GaborApatiNagy/xenadmin,GaborApatiNagy/xenadmin,minli1/xenadmin,minli1/xenadmin,stephen-turner/xenadmin,letsboogey/xenadmin,jijiang/xenadmin,kc284/xenadmin,jijiang/xenadmin,ushamandya/xenadmin,MihaelaStoica/xenadmin,kc284/xenadmin,MihaelaStoica/xenadmin,ushamandya/xenadmin,huizh/xenadmin,jijiang/xenadmin,MihaelaStoica/xenadmin,MihaelaStoica/xenadmin,xenserver/xenadmin,letsboogey/xenadmin,GaborApatiNagy/xenadmin,kc284/xenadmin,stephen-turner/xenadmin,huizh/xenadmin,kc284/xenadmin,geosharath/xenadmin,minli1/xenadmin,geosharath/xenadmin,geosharath/xenadmin,ushamandya/xenadmin,stephen-turner/xenadmin,MihaelaStoica/xenadmin,geosharath/xenadmin,stephen-turner/xenadmin,stephen-turner/xenadmin,huizh/xenadmin,minli1/xenadmin,ushamandya/xenadmin,letsboogey/xenadmin,minli1/xenadmin,jijiang/xenadmin,ushamandya/xenadmin,huizh/xenadmin,jijiang/xenadmin,letsboogey/xenadmin,kc284/xenadmin,xenserver/xenadmin